Slide show: A look inside your eyes

Your eyes are your windows to the world — but they need to be shielded from the elements to keep you seeing clearly. Upper and lower eyelids protect the front of your eyeballs by blocking foreign objects and bright light. In addition, your eyelids involuntarily open and close (blink) every few seconds. During each blink, fluid produced by tear glands passes over the cornea and lubricates the surface of each eye. This helps keep your eyes moist and washes away germs, dust and stray eyelashes.

The front of your eye has three major parts, including the:

• Sclera. This is the white part of your eye. It serves as a tough outer wall that helps maintain your eye's spherical shape and protects its delicate internal structures.
• Pupil. This is the dark spot at the center of your eye. It allows light to pass into the back of your eye.
• Iris. This is the colored part of your eye. It contains a ring of muscle fibers that expand or contract the size of your pupil and control the amount of light entering your eye.

A thin transparent tissue called the conjunctiva covers the sclera. Blood vessels visible in the white part of your eye are located within the conjunctiva.

Behind the scenes, other parts of your eye work to help you see, including the:

• Cornea. This is a protective dome of clear tissue at the front of your eye. It functions as a convex surface that helps focus light rays before they're fine-tuned by the lens.
• Lens. The lens is a clear, elliptical structure about the size and shape of an M&M candy. The curvature of your lens changes to sharpen your focus.
• Vitreous cavity. The vitreous cavity extends from the back of the lens to the back of your eyeball, helping to maintain its shape. This area is filled with a clear, jelly-like substance through which light can pass.

Structures at the back of your eye include the:

• Retina. The retina is a thin layer of tissue that lines the back inner wall of your eyeball. It consists of millions of 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.
• Macula. The macula is a specialized part of the retina located in the center of the back of the eye. This patch of densely packed light-sensitive cells is essential to your central vision and allows you to see fine detail.
• Optic nerve. The optic nerve carries information gathered by your retina to your brain.

Each eyeball has six muscles attached to the sclera — the white part of your eye. These muscles, five of which are shown above, allow you to move your eye and track an object without turning your head. The eye muscles allow you to shift your field of gaze left, right, up, down and diagonally. Your brain coordinates these eye movements so that both eyes can move together when tracking an object.