The iris refers to the eye's colored structure. The iris contains two layers: the iris pigment epithelium (the innermost layer), the iris stroma (the outermost layer). These structural layers contain pigmented cells, which help to synthesize pigment or melanin.
The iris can regulate the light. The muscle tissues help the iris pupil to constrict or dilate, in order to control the light. The pigment cells help to determine the eye's color.
However, a damaged or defective iris often leads to impaired vision. If the iris can not control the light, too much or too less light will enter the eye - this makes the eye unable to focus on the target image. The common iris diseases include: iritis, uveitis, aniridia, iris coloboma.