How the Eye Works
The eye has some parts that are visible and those that aren't and is
an extremely delicate organ. 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.
Eye wall
The wall of a human eyeball is made up of three layers, which are
similar to the layers of skin 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
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 iris is the colored circular membrane in the eye just behind the
cornea. 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 is the soft, light-sensitive layer of tissue that lines
the back of the eyeball wall and 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 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.
Like 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.