The Human Eye

Cornea: the clear outer window window of the eye that covers the iris and pupil that lets light into the eye, thus permitting sight.

Iris: colored or pigmented tissue behind the cornea that acts like a muscular diaphragm, regulating the amount of light the goes into the pupil.

Lens: transparent body behind the iris that is controlled by the ciliary muscle. The iris provides 25 percent of the eye's focusing power. To focus on close-up objects, the ciliary muscle squeezes the lens to make it thicker. For far-away objects, it flattens the lens to make it thinner.

Optic nerve: the bundle of nerve fibers that transmits light-generated electrical impulses from the retina to the brain.

Pupil: the space in the center of the iris where light enters the inner eye. The widening, or dilation, of the pupil is controlled by the iris.

Retina: the inner layer at the back of the eye, where light-sensitive rods and cones are located. Chemical changes in the retina transmit electrical signals through the optic nerve to the brain to produce sight.