Iris & Pupil
Behind the cornea is the iris, the colored part of the eye. The iris acts as a diaphragm, like a shutter on a camera, regulating the amount of light that enters through the pupil. The pupil appears as a black hole at the center of the iris. It is the opening through which light enters the eye. Some iris muscles contract to enlarge the pupil when more light is needed; others reduce it when there is too much light, to protect the inside of the eye. The pupil dilates and constricts in response to changes in lighting but excitement, attractive sights and some drugs also have this effect.
|The iris contracts and expands, adjusting the size of the pupil and
amount of light tht enters the eye.
The amount of pigment in the iris determines the color of the eye. Blue eyes have the least amount of pigment, while brown eyes have the most. The random patterns on the iris are unique to each individual. Each person has a distinct pattern of filaments, pits and striations. Infants' eyes often change color as they become toddlers but from this age forward, the iris color and pattern is stable throughout life.
The iris is protected from external elements by the cornea. It is nourished and cushioned by the aqueous humor, which is produced by and flows from within the iris.
Cassel, G. Billig. The Eye Book: A Complete Guide to Eye Disorders and Health.
Baltimore, MA: Johns Hopkins University Press, 1988.
Collins, J.F. Your Eyes: An Owner's Guide. Englewood Cliffs, NJ: Prentice
D'Alonzo, T.L. Your Eyes: A Comprehensive Look at the Understanding and
Treatment of Vision Problems. Clifton Heights, PA: Avanti Publishing, 1991.
Eden, J. The Physician's Guide to Cataracts, Glaucoma, and Other Eye Problems.
New York, NY: Consumer Reports Books, A Division of Consumers Union Yonkers,
Schuman, B.N. The Human Eye. New York, NY: Atheneum, 1986.
Adler, R., Adler, I. Your Eyes. New York, NY: The John Day Company,
Begbie, G.H. Seeing and the Eye: an Introduction to Vision. Garden City,
NY: National History Press, 1996.
Cohen, N.S. Out of Sight Into Vision: There is More to Good Vision Than
Reading the Fine Print. Toronto, Canada: Collier Macmillan Canada, 1997.
Kwiko, M.L. Eyes. Toronto, Canada: Key Porter Books, 1994.
Rainwater, J. Vision: How, Why, and What We See. New York, NY: Golden