The iris is the colored part of the eye. It is located between the cornea and lens. The round, central opening of the iris is called the pupil. Very small muscles in the iris cause the pupil to get smaller and bigger to control how much light comes into the eye. This allows you to see well in lighter and darker settings.
The number of color granules in the iris determines the iris color (pigment). The amount of pigment is often related to a person's genes, skin type, and hair color.
Inflammation of the iris is called iritis. Rarely, tumors can form in the iris. When tumors do form, most often they are melanoma.
Evans M. Anatomy of the uvea. In: Yanoff M, Duker JS, eds. Ophthalmology. 4th ed. Philadelphia, PA: Elsevier Saunders; 2014:chap 7.1.
Jung CS, Grossniklaus HE, Hutchinson AK. Iris. In: Tasman W, Jaeger EA, eds. Duane's Foundations of Clinical Ophthalmology on CD-ROM - 2013 Edition. Philadelphia, PA: Lippincott Williams & Wilkins; 2013: vol 1, chap 11.