CORNEA: Transparent front segment of the eye that covers iris, pupil, and anterior chamber, and provides most of an eye's optical power.

PUPIL: Variable-sized, circular opening in center of iris; it appears as a black circle and it regulates the amount of light that enters the eye.

IRIS: Pigmented tissue lying behind cornea that (1) gives color to the eye, and (2) controls amount of light entering the eye by varying size of black pupillary opening; separates the anterior chamber from the posterior chamber.

LENS: Natural lens of eye; transparent intraocular tissue that helps bring rays of light to focus on the retina.

RETINA: Part of the eye that converts images into electrical impulses sent along the optic nerve for transmission back to the brain. Consists of many named layers that include rods and cones.

MACULA: Small, specialized central area of the retina responsible for acute central vision.

VITREOUS: Transparent, colorless, gelatinous mass; fills rear two-thirds of the interior of the eyeball, between the lens and the retina.

OPTIC NERVE: Largest sensory nerve of the eye; carries impulses for sight from retina to brain.