Eyelids

Upper and lower eyelids protect the front of your eyeball (orbit) by blocking 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.

The front of your eye

Three parts make up the front of your eye, or the part of your eye that's visible to others:

• 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 the conjunctiva.

Your inner eye

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.

The back of your eye

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.

• Macula. Your macula is a specialized part of the retina. This patch of densely packed light-sensitive cells is essential to your central vision and allows you to see fine detail.

• Optic nerve. Your optic nerve carries information gathered by your retina to your brain via a bundle of more than 1 million nerve fibers.

How your eyes move

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.