iris

the part of the EYE that controls the amount of light that enters. It is, in effect, a muscular disc, and to reduce the amount of light entering, circular MUSCLES contract, and to increase the aperture in dim light, radiating muscles contract. The varying-sized hole is the PUPIL . The iris can be seen through the CORNEA , which is transparent and is the coloured part of the eye. This latter feature is accounted for by PIGMENT cells containing melanin (blue is little; brown is more).