The iris is located between the anterior and posterior eye chamber and has a circular shape. The ciliary body exits from the outer edge of the iris, and the inner edge passes into the pupil. The iris consists of six layers, which include the small and large iris ring, pigment cells, arteries, smooth muscles and ligaments. The smooth muscle of the iris consists of two muscles that control pupil size: sphincter of the pupil and pupil splitter. These muscle reflexes are conditioned by the amount of light coming upon eye. If there are low light conditions, the pupil expands (Mydriasis), on the contrary, in better light conditions, the pupil shrinks (Meiosis). The color of the iris is determined genetically. In exceptional cases, it can happen that each human eye iris has a different color. This condition is called heterochromia and is caused by an excess or lack of melanin pigment in the affected eye. Heterochromia may be either complete (each eye a different color), or partial where only part of the eye has discoloration.