Which part of your body lets you read the back of a cereal box,
check out a rainbow, and see a softball heading your way? Which
part lets you cry when you're sad and makes
to protect itself? Which part has muscles that adjust to let you
focus on things that are close up or far away? If you guessed the
eye, you're right!
Your eyes are at work from the moment you wake up to the moment
you close them to go to
. They take in tons of information about the world around you -
shapes, colors, movements, and more. Then they send the information
to your brain for processing so the
knows what's going on outside of your body.
You can see that the eye's pretty amazing. So, come on -
let's take a tour of its many parts.
You can check out different parts of the eye by looking at your
own eye in the mirror or by looking at (but not touching) a
friend's eye. Some of the eye's parts are easy to see, so
most friends will say OK. Most friends won't say OK if you ask
to see their liver!
The eye is about as big as a ping-pong ball and sits in a little
hollow area (the
) in the skull. The front part is protected by the
. The eyelid helps keep the eye clean and moist by opening and
shutting several times a minute. This is called
, and it's both a voluntary and involuntary action, meaning you
can blink whenever you want to, but it also happens without you
even thinking about it.
The eyelid also has great reflexes, which are automatic body
responses, that protect the eye. When you step into bright light,
for example, the eyelids squeeze together tightly to protect your
eyes, until they can adjust to the light. And if you flutter your
fingers close (but not too close!) to your friend's eyes,
you'll be sure to see your friend's eyes blink. Your
friend's eyelids shut automatically to protect the eye from
possible danger. And speaking of fluttering, don't forget
. They work with the eyelids to keep dirt and other unwanted stuff
out of your eyes.
The white part of the eyeball is called the
-uh). The sclera is made of a tough material and has the important
job of covering most of the eyeball. Think of the sclera as your
eyeball's outer coat. Look very closely at the white of the
eye, and you'll see lines that look like tiny pink threads.
These are blood vessels, the tiny tubes that deliver blood, to the
The cornea, a transparent dome, sits in front of the colored
part of the eye. The cornea (say:
-nee-uh) helps the eye focus as light makes its way through. It is
a very important part of the eye, but you can hardly see it because
it's made of clear tissue. Like clear glass, the cornea gives
your eye a clear window to view the world through.
Behind the cornea are the iris, the pupil, and the anterior
chamber. The iris (say:
-riss) is the colorful part of the eye. When we say a person has
blue eyes, we really mean the person has blue irises! The iris has
muscles attached to it that change its shape. This allows the iris
to control how much light goes through the pupil (say:
The pupil is the black circle in the center of the iris, which
is really an opening in the iris, and it lets light enter the eye.
To see how this works, use a small flashlight to see how your eyes
or a friend's eyes respond to changes in brightness. The pupils
will get smaller when the light shines near them and they'll
open wider when the light is gone.
The anterior (say: an-
-ee-ur) chamber is the space between the cornea and the iris. This
space is filled with a special transparent fluid that nourishes the
eye and keeps it healthy.
Light, Lens, Action
These next parts are really cool, but you can't see them
with just your own eyes! Doctors use special microscopes to look at
these inner parts of the eye, such as the
. After light enters the pupil, it hits the
The lens sits behind the iris and is clear and colorless. The
lens' job is to focus light rays on the back of the eyeball - a
part called the
-tin-uh). The lens works much like the lens of a movie projector at
the movies. Next time you sit in the dark theater, look behind you
at the stream of light coming from the projection booth. This light
goes through a powerful lens, which is focusing the images onto the
screen, so you can see the movie clearly. In the eye's case,
however, the film screen is your retina.
Your retina is in the very back of the eye. It holds millions of
cells that are sensitive to light. The retina takes the light the
eye receives and changes it into nerve signals so the brain can
understand what the eye is seeing.
The lens is suspended in the eye by a bunch of fibers. These
fibers are attached to a muscle called the
. The ciliary muscle has the amazing job of changing the shape of
the lens. That's right - the lens actually changes shape right
inside your eye! Try looking away from your computer and focusing
on something way across the room. Even though you didn't feel a
thing, the shape of your lenses changed. When you look at things up
close, the lens becomes thicker to focus the correct image onto the
retina. When you look at things far away, the lens becomes
The biggest part of the eye sits behind the lens and is called
. The vitreous body forms two thirds of the eye's volume and
gives the eye its shape. It's filled with a clear, jelly-like
material called the
. Ever touch toy eyeballs in a store? Sometimes they're kind of
squishy - that's because they're made to feel like
they're filled with vitreous humor. In a real eye, after light
passes through the lens, it shines straight through the vitreous
humor to the back of the eye.
Rods and Cones
The retina uses special cells called rods and cones to process
light. Just how many rods and cones does your retina have? How
about 120 million rods and 7 million cones - in each eye!
see in black, white, and shades of gray and tell us the form or
shape that something has. Rods can't tell the difference
between colors, but they are super-sensitive, allowing us to see
when it's very dark.
and they need more light than rods to work well. Cones are most
helpful in normal or bright light. The retina has three types of
cones. Each cone type is sensitive to one of three different colors
- red, green, or blue - to help you see different ranges of color.
Together, these cones can sense combinations of light waves that
enable our eyes to see millions of colors.
Rods and cones process the light to give you the total picture.
You're able to see that your friend has brown skin and is
wearing a blue hat while he tosses an orange basketball.
Sometimes someone's eyeball shape makes it difficult for the
cornea, lens, and retina to work perfectly as a team. When this
happens, some of what the person sees will be out of focus.
To correct this fuzzy vision, many people, including many kids,
. Glasses help the eyes focus images correctly on the retina and
allow someone to see clearly. As adults get older, their eyes lose
the ability to focus well and they often need glasses to see
things up close or far away. Most older people you know - like your
grandparents - probably wear glasses.
To the Brain!
Think of the
as the great messenger in the back of your eye. The rods and cones
of the retina change the colors and shapes you see into millions of
nerve messages. Then, the optic nerve carries those messages from
the eye to the brain! The optic nerve serves as a high-speed
telephone line connecting the eye to the brain. When you see an
image, your eye "telephones" your brain with a report on
what you are seeing so the brain can translate that report into
"cat," "apple," or "bicycle," or
whatever the case may be.
Have No Fear, You Have Tears
For crying out loud, the eye has its own special bathing system
- tears! Above the outer corner of each eye are the
, which make tears. Every time you blink your eye, a tiny bit of
tear fluid comes out of your upper eyelid. It helps wash away
germs, dust, or other particles that don't belong in your eye.
It also keeps your eye from drying out. Then the fluid drains out
of your eye by going into the
(this is also called the tear duct). You can see the opening of
your tear duct if you very gently pull down the inside corner of
your eye. When you see a tiny little hole, you've found the
Your eyes sometimes make more tear fluid than normal to protect
themselves. This may have happened to you if you've been poked
in the eye, if you've been in a dusty or smoking area, or if
you've been near someone who's cutting onions.
And how about the last time you felt sad, scared, or upset? Your
eyes got a message from your brain to make you cry, and the
lacrimal glands made many, many tears.
Your eyes do some great things for you, so take these steps to
- Wear goggles in classes where debris or chemicals could go
flying, such as wood shop, metal shop, science lab, or art.
- Wear eye protection when playing racquetball, hockey, skiing,
or other sports that could injure your eyes.
- Wear sunglasses. Too much light can damage your eyes and
cause vision problems, such as
, later in life. If the lens gets cloudy, it's called a
cataract. A cataract prevents light from reaching the retina and
makes it difficult to see.
The eyes you have will be yours forever - treat them right and
they'll never be out of sight!
Elana Pearl Ben-Joseph, MD
Date reviewed: November 2006
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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