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.
Sources
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Understanding and Treatment of Vision Problems.
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Eden, J. The Physician's Guide to Cataracts,
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Consumer Reports Books, A Division of Consumers Union
Yonkers, 1992.
Schuman, B.N. The Human Eye. New York, NY:
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Adler, R., Adler, I. Your Eyes. New York, NY: The
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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,
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Rainwater, J. Vision: How, Why, and What We See.
New York, NY: Golden Press, 1992. |
