Encyclopedia - Human Eye

Your eye, an organ about 1 inch in diameter, allows you to see and interpret shapes, colors, and dimensions of objects. It works by changing reflected light from the environment into electrical signals, which are sent to the brain via the optic nerve (See Anatomy of the Eye). The cone-shaped cavity in your skull where your eye is located is called the orbit or socket. Surrounded by layers of soft, fatty tissue, the orbit protects your eye and enables it to turn easily. Your eye is comprised of nine parts:

Cornea -- Transparent front covering of your eye that serves as the outer lens and provides about 70 percent of your eye's focusing power. The cornea is made up of five layers of tissue: epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium. The epithelium is the outer, protective layer. It heals from superficial injuries in about three days. A cornea that is less than perfect in shape causes most vision problems that are corrected with eyeglasses, contact lenses, or refractive surgery.
Conjunctiva -- Transparent mucous membrane that covers the outer surface of the eyeball, except the cornea, and lines the inner surfaces of the eyelids. Tiny glands in the conjunctiva secrete tear film to lubricate the surface of the eye.
Iris -- Pigmented tissue lying behind the cornea, which determines eye color. The iris controls the amount of light entering your eye by contracting or expanding the size of your pupil. Like the shutter in an automatic camera, your pupil gets larger in low light situations and smaller when there is bright light.
Lens -- Natural crystalline lens of your eye that lies behind the pupil and, by changing its shape, works to fine-tune light images before they strike the retina. As you mature, this lens begins to lose its elasticity causing presbyopia, which is difficulty in focusing on near objects. You may need reading glasses or bifocals to correct this condition. When this lens becomes cloudy, you have a cataract.
Optic nerve -- Nerve that begins near the center of the back of the eye and is responsible for carrying electrical signals created by the retina to the brain. Damage to this nerve, such as that caused by glaucoma, can seriously impair vision.
Pupil -- Variable-sized black circular opening in the center of the iris that works like the shutter in a camera to regulate the amount of light that enters the eye. The pupil is normally small in bright light and large in dim light. When an eye doctor dilates your eye with special eye drops, your pupil opens wider, allowing the doctor to view structures on the inside of your eye more easily.
Retina -- Thin layer of light-sensitive nerve tissue that lines the inside of the back two-thirds of the eye. The retina acts like the film in a camera by receiving light signals from the outside world through the cornea, the pupil, and the natural crystalline lens. Specialized cells in the retina, called rods and cones, convert this light energy into electrical impulses that are transmitted to the brain through the optic nerve.
Sclera -- The tough, outermost layer of the eye that is visible as the white of the eye. The sclera provides structure, strength, and protection to the eye.
Vitreous body -- Clear, gel-like substance that fills the inner cavity of the eye and sustains its shape. When small flecks of protein or other natural matter become trapped in the vitreous, they are sometimes visible as floaters. Floaters are normally harmless, although sometimes annoying.