The most visibly striking feature of the human eye is the iris, the colorful part of the eye encircling the pupil. The pupil is the black “donut hole” in the center of the iris. On a microscopic level, the iris is a beautifully weaved, textured mesh of peaks and valleys.
How the Iris Works:
Like a louvered window shade, the amount of light entering the eye is regulated by the iris controlling the size of the pupil. More or less light is allowed into the eye by how wide (dilated) or narrow (constricted) the iris tissue becomes. There are two muscles that reside in the iris which act in opposition. One is a highly-specialized muscle that causes the iris to flatly stretch out and make the pupil smaller when light conditions are bright.
The opposing muscle is a “radial” muscle. Anatomically, this radial muscle is spread out through the iris like the spokes of a round bicycle wheel. Under contraction, the radial muscle fibers cause the iris tissue to bunch up, similar to window drapes bunching up when fully opened. This enlarges the pupil, allowing more light to enter the eye in dim or dark illumination.
Beautiful, Functional, & Colorful
The human iris is amazingly different, from individual to individual. Most people have colors ranging from blue, green, gray, hazel, amber, or brown, or some shade between. In rare cases, people will even have red or violet irises, typically as a side effect of a genetic mutation accompanying albinism. Each person has a perfectly unique pair of eyes, caused by the variations in color and due to the unique textured mesh of muscle comprising the iris itself.
Some people actually have two different iris colors, one for each eye, due to a different genetic anomaly called heterochromia. This mutation also accounts for different colors within the same iris (which you can see in a few of the colors above).
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