The IRIS is the colorful part of the eye, located between the white sclera and the pupil. It is a thin, circular structure that is made up of muscle fibers and pigmented cells. The color of the IRIS is determined by the amount of melanin present in the cells. The size of the pupil is regulated by the muscles in the IRIS, which contract or expand to control the amount of light entering the eye. The IRIS also helps to protect the eye from damage caused by bright light.