Eyes

Eyes: Small But Mighty

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.

The Basics on Eyes

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 our 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 moisture over the cornea (pronounced: kor-nee-uh), which covers the front of the eye. The lacrimal (pronounced: lah-kruh-mul) 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 that leads to the nose.

For you to see, the eye has to move. Six muscles, called extraocular (pronounced: ek-struh-ah-kyuh-lur) 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 (pronounced: skler-uh) 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. The conjunctiva, a clear mucous membrane that protects the eye from becoming dry, sits over the sclera and also covers the inner surface of the eyelid.
• The choroid (pronounced: kor-oyd) is the middle layer that contains blood vessels that deliver oxygen and nutrients to the retina.
• The retina (pronounced: ret-nuh), 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 (pronounced: vih-tree-us) humor. This material allows light to pass through to the retina. It also helps the eye keep its round shape.