The iris is the aperture of the eye. It constricts in bright light to limit the light entering the eye, and it dilates in dark environments to maximize the light entering the eye.
While the iris appears innocent enough, it is often involved in many diseases of the eye. It can be become inflammed and cause significant pain in arthritic-like conditions. It can grow new blood vessels and cause severe pressure rise in the eye in diabetes. It can degenerate or be absent altogether in some congenital disorders. It can grow tumors on both the front and the back.