Eyes


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:

1. 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.
   
2. The choroid is the middle layer that contains blood vessels that deliver oxygen and nutrients to the retina.
   
3. 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.