As you look at this magazine, light is reflected off the page and onto your CORNEA (1) , a windowlike covering that acts like the glass of a camera lens. The adult cornea is only about half a millimeter thick yet contains more nerve endings than any other place in the body.
The LENS (2) focuses, bends, and refracts the light. Crystalline and fragile, it's suspended within the eye by tiny guy wires called ZONULES (3) . The younger you are, the better your lens is able to change shape to adjust for near or distant vision. As you age, the lens stiffens like an arthritic joint, which causes some of the eye problems people commonly experience after age 50 or so.
The colored part of your eye is the IRIS (4) . Eye color is determined by microscopic pigment cells called MELANIN (5) . The color, texture, and patterns of each person's irises are as unique as fingerprints.
The iris is embedded with tiny muscles that dilate to constrict the PUPIL (6) , which, like a camera's aperture, regulates the amount of light that enters. When it's dark outside, your pupils expand to gather as much light as possible. In bright light the pupil constricts.
After light passes through the pupil, it lands on the RETINA (7) , a multilayer sensory tissue that lines the back of the eye. The retina acts like the film in a camera. It contains millions of PHOTORECEPTORS (8) that capture light rays and convert them into electrical impulses, which are sent to the brain via the optic nerve. The brain develops those impulses into the images you perceive.
Eye movement is controlled by the EXTRAOCULAR MUSCLES (9) , a collection of six muscles that direct your eyesight left to right and up and downhelping you track a ball or scan downriver.
EYELIDS (10) and eyelashes are your first line of defense against dirt and debris. With each blink, the eyelids spread a film of oil, water, and mucus evenly across the eye's surface.