The iris is part of the:

Explanation

At the junction of the sclera and the cornea, the vascular coat sharply bends into the cavity of the eyeball to form a thin, coloured partition. This partition is called iris. It is perforated at the middle by an aperture called the pupil. The iris contains two sets of smooth muscles: sphincters and dilators. These muscles regulate the amount of light entering the eyeball by varying the size of the pupil. The sphincter muscles are arranged in rings. Their contraction makes the pupil smaller in bright light so that less light enters the eye. The dilator muscles are arranged radially. Their contraction widens the pupil in dim light to let in more light. Iris, by regulating the size of the pupil, allows light to pass only through the centre of the lens, which is optically the most effective part.