| MadSci Network: Anatomy |
The choroid, ciliary body and the iris all form the middle layer of tissues in the eye, from the back to the front. This layer also contains different eye pigments. The iris at the front of the eye creates your pupil and it contains cells that make different amounts of pigment in different peoples.
Your eyes let light in , but also there is a reflex of light or a reflection of some of the light back off your retina. There is alot of red in your eye, for example, due to the blood supply in your retina. Your retina also has some layers of cells that make pigments as well. Generally these pigments are protective, like a shade, and help adsorb some of the light radiation that impinges upon your retina.
Persons with distant ancestry from brighter, sunny portions of the earth near the equator tend to have darker and browner skin, and eyes which are more protective against the ravages of prolonged sun exposure. Blond, blue-eyed persons have less eye pigments in the iris. Your exact eye color is a result of light reflecting off your iris, light shining through it, and even pupil size which thickens and thins the iris. Light also reflects and transmits differently though your cornea and sclera (white part of eye) depending on how wet it is, and how red your eyes may be from crying. So your eye colour may seem different at times. Also the pigment of the iris is really patchy if you look closely, so you may notice more yellow flecks in hazel eyes sometimes when close, but from farther away the iris may just seem a smooth even green. Most babies are born with blue eyes, and over several weeks they may get green to brown eyes as their iris cells make pigments.
Ken Mitton, PhD(UWO94) Kellogg Eye Center Dept of Ophthalmology & Visual Sciences University of Michigan kpmitton@umich.edu
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