iris is a topic covered in the Taber's Medical Dictionary.

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(ī′rĭs )

pl. irides [Gr. iris, stem irid-, the goddess Iris, the messenger of the Olympian gods; rainbow; iris (plant)]
The opaque, pigmented continuation of the choroid layer of the eye that partly covers the lens, surrounding the pupil and regulating the amount of light that strikes the retina. The iris lies in the aqueous compartment of the eye. It divides the posterior chamber of the aqueous compartment (into which aqueous fluid is secreted) and the anterior chamber (from which aqueous fluid is absorbed). Muscles in the iris can expand or contract the pupil.
SEE: aniridia; SEE: choroidoiritis; SEE: heterochromia iridis; SEE: rubeosis iridis

The front surface of the iris is irregular and covered with grooves and ridges. The posterior surface contains radially oriented myofilaments, which together form the dilator pupillae muscle (which widens the pupil in response to sympathetic stimulation). A separate set of concentric myofilaments forming circles around the pupil compose the sphincter pupillae muscle (which narrows the pupil in response to parasympathetic stimulation). The color of the iris is determined by the concentration of melanocytes: if there are few cells, the color will be blue; increasing concentrations of melanocytes lead to darkening color on a spectrum from greenish-blue, to gray, to brown.

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