All About Eyes
We depend on sight more than any other of our senses to maneuver
through the space around us. In a single glance, lasting a fraction
of a second, our eyes work with our brains to tell us the size,
shape, color, and texture of an object. They let us know how close
it is, whether it's standing still or coming toward us, and how
quickly it's moving. Every day, our eyes give us messages that
help us understand the world around us.
Although the eyes are small compared with most of the body's
other organs, their structure is incredibly complex. The eyes work
together to perceive depth, enabling us to judge distance and the
size of objects to help us move around them. Not only do the two
eyes work together, they also work with the brain, muscles, and
nerves to produce complicated visual images and messages. And our
eyes constantly adapt to the changing environment - for example,
they are able to adjust so that we can easily move around in a
nearly dark room or bright sunlight.
To understand more about how the eyes work, it's important
to know about the structures that make up the eye and about
conditions and diseases that can interfere with vision.
How Eyes Work
Only part of the eye is visible in a person's face. The
whole eye - the
eyeball
- is about the size and shape of a ping-pong ball.
The eye - both the parts that are visible and those that
aren't - is extremely delicate. The body has several ways of
protecting this vulnerable organ. The eyeball sits in the eye
socket (also called the
orbit
) in a person's skull, where it is surrounded by bone. The
visible part of the eye is protected by the eyelids and the
eyelashes, which keep dirt, dust, and even harmful bright light out
of the eye.
Our eyes are also protected by tears, which moisten the eyes and
clean out dirt, dust, and other irritants that get past the
defenses of our eyelashes and eyelids. Tears also help protect the
eyes against infection.
Every time we blink, our eyelids spread a layer of mucus, oil,
and tears over the cornea, which covers the eye. The
lacrimal
glands
in the upper outer corner of each eye socket produce tears.
After they've done their job moistening the eyes, the tears
flow into canals in the eyelids, which drain into the
lacrimal sac
, a pouch in the lower inner corner of each eye socket. Tears then
exit through a passage which leads to the nose.
To see, the eye has to move. Six muscles, called
extraocular
muscles
, surround the eyeball in the skull. These muscles act like the
strings on a puppet, moving the eye in different directions. The
muscles of each eye normally move together at the same time,
allowing the two eyes to remain aligned.
The wall of a person's eyeball is made up of three layers,
rather like the layers on an onion:
- The
sclera
is the outermost protective layer. This tough, fibrous tissue
surrounds the eyeball and attaches to the cornea, which is the
clear front surface of the eye. What we see as the white of the
eye is the sclera. Over the sclera lies the conjunctiva, a clear
mucous membrane that protects the eye from becoming dry.
- The
choroid
is the middle layer that contains blood vessels that deliver
oxygen and nutrients to the retina.
- The
retina
, the innermost of the three layers, lines the inside of the
eyeball. The retina is a soft, light-sensitive layer of nervous
system tissue. The optic nerve carries signals from the retina to
the brain, which interprets them as visual images.
The space in the center of the eyeball is filled with a clear
jelly-like material called the
vitreous
humor
. This material allows light to pass through to the retina. It also
helps the eye keep its round shape.
Vision is the process by which images captured by the eye are
interpreted by the brain, and the visible part of the eye is where
the process of sight begins. On the front surface of the eye is the
see-through, circle-shaped cornea. You can't see a person's
cornea the way you can see the colored part of the eye behind it -
the cornea is like a clear window that focuses light into the eye.
Behind the cornea is a watery fluid called the
aqueous
humor
. The cornea and aqueous humor form an outer lens that
refracts
(bends) light on its way into the eye. This is where most of the
eye's focusing work is done.
The colored circular membrane in the eye just behind the cornea
is called the
iris
. The iris controls the amount of light entering the eye through
the
pupil
, which is the opening in the center of the iris that looks like a
tiny black circle. Like a camera, which controls the amount of
light coming in to prevent both overexposure and underexposure, the
iris becomes wider and narrower, changing the size of the pupil to
control the amount of light entering the eye. The pupil gets bigger
when more light is needed to see better and smaller when
there's plenty of light.
The eye's
lens
sits just behind the iris. Just like a camera lens, the eye's
lens focuses light to form sharp, clear images. Light that has been
focused through the cornea and aqueous humor hits the lens, which
then focuses it further, sending the light rays through the
vitreous humor and onto the retina.
To focus on objects clearly at varying distances, the eye's
lens needs to change shape. The
ciliary
body
contains the muscular structure in the eye that changes the shape
of the eye's lens. In people who have normal vision, the
ciliary body flattens the lens enough to bring objects into focus
at a distance of 20 feet or more. To see closer objects, this
muscle contracts to thicken the lens. Young children can see
objects at very close range; many people over 45 have to hold
objects farther and farther away to see them clearly. This is
because the lens becomes less elastic as we age.
The retina (the soft, light-sensitive layer of tissue that lines
the back of the eyeball wall) is made up of millions of light
receptors. These are called rods and cones. Rods are much more
sensitive to light than cones. Each eye has about 125 million rods
that help us see in dim light and detect shades of gray, but they
cannot distinguish colors. In comparison, the 6 million cones in
each eye allow us to see in bright light, and they also sense color
and detail.
The
macula
is a small, specialized area on the retina. The macula helps our
eyes see fine details when we look directly at an object. It
contains mainly cones and few rods.
When focused light is projected onto the retina, it stimulates
the rods and cones. The retina then sends nerve signals are sent
through the back of the eye to the
optic nerve
. The optic nerve carries these signals to the brain, which
interprets them as visual images. The portion of the brain that
processes visual input and interprets the messages that the eye
sends is called the
visual cortex
.
As in a camera, the eye's lens transmits light patterns
upside down. The brain learns that the impulses received from the
upper part of the retina are really from the lower part of the
object we're seeing and vice versa.
Most people use both eyes to see an object. This is called
binocular vision. Through binocular vision, images are formed on
the retina of each eye. These images are slightly different,
because the object is being viewed from slightly different angles.
Nerve signals representing each image are sent to the brain, where
they are interpreted as two views of the same object. Some of the
nerve fibers from each eye cross, so each side of the brain
receives messages from both eyes. Through experience, the brain
learns to judge the distance of an object by the degree of
difference in the images it receives from the two eyes. This
ability to sense distance is called depth perception.
Vision is a fine-tuned process. All the parts of the eye - and
the brain - need to work together so a person can see correctly.
Because the eye's structure is so complex, though, a lot of
things can go wrong.
Vision Problems
Some of the most common eye problems are
refractive errors
. These are the problems that eye doctors check for routinely in a
vision test. Refraction means bending of light rays to focus the
light coming from an image. Refractive errors are problems with the
focusing of the eye, because of the way the eye is shaped, which
causes the image you see to be blurred.
Refractive errors include:
-
Astigmatism.
In astigmatism, there's a problem with the curve of the
cornea. This causes part of the eye's image to be blurry.
Corrective lenses such as contact lenses or glasses can usually
correct vision in people with astigmatism.
-
Nearsightedness.
Also called
myopia
or shortsightedness, nearsightedness occurs when the eye focuses
the image of an object in front of the retina instead of directly
on it. In most cases, people can't see well far away, but can
see objects clearly close up. The condition tends to get somewhat
worse through childhood and adolescence, but stabilizes in
adulthood. People with this condition may need to wear glasses or
contacts to correct their vision. Laser eye surgery is now
commonly used in adults to correct nearsightedness permanently by
changing the shape of the cornea. Laser surgery is not used in
kids and teens because the eye may still be growing and the
refractive error changing.
-
Farsightedness.
Also called
hyperopia
or longsightedness, farsightedness occurs when the incoming image
is not focused on the retina, but behind it. This makes it
difficult to see close objects clearly, even though far-off
objects can be seen clearly. Glasses or contact lenses can
correct this problem in kids and teens. Most adults develop a
form of farsightedness called
presbyopia
as they get older.
Some of the other eye problems that can affect children and
teens include:
Blindness.
This is the loss of useful sight. Blindness can be temporary or
permanent and it has many causes. Birth defects or damage to any
portion of the eye, the optic nerve, or the area of the brain
responsible for vision can lead to blindness. The visual impairment
cannot be corrected with surgery or corrective lenses, and the
condition can make it difficult to perform everyday activities.
Diabetes, problems with the macula, and eye changes because of
aging account for most blindness in the United States.
Cataracts.
Some children are born with cataracts, a cloudiness of the
eye's lenses that prevent images from being seen clearly or at
all. But cataracts are much more prevalent in the elderly among
whom surgery to remove cataracts and correct vision is common. In
infants and young children, cataracts must be treated to prevent
permanent problems with development of vision.
Color blindness.
Color blindness is caused by problems in the pigments of the cones
in the retina. Most people who are color-blind can see some colors.
In most cases, someone who is color-blind confuses some colors with
others, usually red and green. People can be born color-blind or
they may develop the condition over time. The most common form of
color blindness is an inherited condition that affects boys much
more often than girls. Color blindness cannot be corrected.
Conjunctivitis.
Conjunctivitis refers to inflammation (redness, pain, and swelling)
of the conjunctiva. One type of conjunctivitis is called pinkeye, a
common contagious infection in which the eyes become pinkish red
and watery, and pus may form. Pinkeye is usually treated with
eyedrops. Conjunctivitis may also occur due to allergies or from a
scratch on the surface of the eye.
Dacryostenosis.
Dacryostenosis is a blockage of the tear drainage system of the eye
occurring in the lower eyelid. This is a fairly common problem in
infants, but often improves with time.
Eye injuries.
Injuries to the eye are one of the most common preventable causes
of blindness. Eye injuries may be caused by irritants such as sand,
dirt, or other foreign bodies on the eye's surface. Chemicals
or foreign bodies that become embedded in the eye can also cause
pain and loss of vision. Forceful blows to the eye can cause
bleeding inside the eye and damage to the cornea, retina, and other
important eye structures.
Glaucoma.
One of the leading causes of blindness in the United States,
glaucoma is a disorder that causes fluid pressure to build up
inside the eye, potentially causing damage to the optic nerve.
Although it can occur in infants and children, it is much more
common among older adults. Unless detected early by routine
screening tests, it usually goes undiagnosed until some vision is
lost.
Macular degeneration
is an eye disease that mainly affects older people. But sometimes
infants and children can develop it, too. When someone has macular
degeneration, the middle area of the retina becomes scarred. This
causes eyesight to deteriorate over many years. The cause of
macular degeneration is unknown in most cases, but it tends to run
in families, which indicates that genes and heredity are
involved.
Retinoblastoma.
This is a cancerous tumor in the eye that usually appears in the
first 5 years of life. There may be permanent vision loss, and the
affected eye(s) may have to be removed.
Retinopathy
of prematurity.
Premature babies sometimes have damage to the blood vessels in the
eye's retina, a condition known as retinopathy of prematurity,
which can lead to severe vision loss in some cases.
Strabismus.
A condition in which the eyes appear crossed, strabismus is usually
caused by a defect in the balance of the pull exerted by the
muscles that help the eyes stay straight and move together
properly. Surgery on the eye muscles can help realign the eyes.
Permanent vision loss in the less-used eye (
amblyopia
) can occur if the condition is not treated in early childhood.
Sty.
A sty is an infection of one of the follicles from which the
eyelashes grow. Sties are treated by applying warm compresses and
using antibiotics.
Proper care of your child's eyes includes having periodic
eye exams at routine office checkups and by an eye specialist
according to the schedule recommended by your doctor. Signs that a
very young child may be having vision trouble include constant eye
rubbing, tearing or redness, and trouble following an object. Older
kids may squint or sit too close to the TV. Kids who are having
trouble seeing might also complain of headaches at the end of the
day.
Reviewed by:
Steven Dowshen, MD
Date reviewed: April 2008
Originally reviewed by:
Alexander E. Pogrebniak, MD
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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