Teri Gravitte, age 11, of Richmond, Va., for her question:

WHAT DOES THE IRIS IN THE EYE DO?

At the very front of the eye, and actually located in the middle layer, is the iris. The iris is a thin curtain of tissue in front of the lens. Light passes through the pupil, which is a round hole in the iris. The pupil looks a lot like a black circle.

Two sets of muscles in the iris change the size of the pupil. This controls the amount of light that enters the eye. When examining the inside of the eye, a doctor will sometimes use drugs in eye drops to dilate or enlarge the pupil.

The amount and location of pigment in the iris determine whether the eye looks blue, brown, green or gray.

An inflammation of the iris is called iritis.