Slide show: A look inside your eyes

Your eyes are a small part of your body — each eyeball is about an inch in diameter. Yet your eyes play a big role in your life. With them you experience the shape, color and motion of your surroundings. They alert you to danger or to the unexpected. You rely on them to help you explore and learn.

Here's a look at the various parts of the eye and how they work together to help you see the world around you. Each part plays an essential role in your eyes' healthy functioning.

Upper and lower eyelids protect the front of your eyeball (orbit) by blocking out foreign objects and bright light that can damage your eye. Your eyelids involuntarily open and shut (blink) every few seconds to help protect your eyeball. You can, of course, blink intentionally as well.

With each blink, a tiny bit of tear fluid comes out of a gland near each eye. The lubricant washes away germs, dust, eyelashes or other foreign objects and keeps your eyes from drying out.

Three parts make up the front of your eye:

• Sclera. The white of your eye is actually a tough, leathery coating that helps form the spherical shape of your eyeball and protects the delicate internal structures of your eye.
• Pupil. The dark spot at the center of your eye is a hole that allows light to pass into your eye.
• Iris. The colored part of your eye contains a ring of muscle fibers that can expand or contract the size of your pupil, controlling the amount of light entering your eyeball.

A thin transparent tissue called the conjunctiva covers the sclera of your eye. The blood vessels that are often visible in the white part of your eye run through your eye's conjunctiva.

Behind the scenes, other parts of your eye are working to help you see:

• Cornea. Your cornea is a protective dome of clear tissue over the front of your eye. It functions as a convex surface that helps focus the light rays before they are fine-tuned by the lens.
• Lens. Your lens is a clear, elliptical structure about the size and shape of an M&M'S candy. The curvature of your lens changes to sharpen the focus of whatever you're looking at.
• Vitreous cavity. The vitreous cavity extends from the back of the lens to the back of your eyeball. It's filled with a clear, jelly-like substance that allows light to pass through. It also maintains the shape of your eyeball.

Structures at the back of your eye help complete the vision process:

• Retina. Your retina is a thin layer of tissue that lines the back inner wall of your eyeball. Your retina consists of millions of light-sensitive cells and nerve cells that capture the images focused onto them by your cornea and lens. When light hits these cells, electrical impulses are generated and carried to your optic nerve.
• Optic nerve. Your optic nerve carries information gathered by your retina to your brain via a bundle of more than 1 million nerve fibers.

Each eyeball has six muscles attached to the sclera — five are seen from this angle — allowing you to move both eyes and track an object without necessarily turning your head. These eye muscles, working individually or together, allow you to shift your field of gaze left, right, up, down and diagonally. Your brain coordinates these eye movements, so your eyes move together when tracking an object.