Explanation

Middle layer of eye is choroid, which on anterior side becomes thick from ciliarybody the ciliary body itself continuous forward to form iris. In front of lens the aperture surrounded by the iris is called pupil. In the middle of a normal iris, pupil can be seen. It is an opening that is circular and is comparable to the aperture of a camera. As the amount of light entering the eye diminishes (such as in a dark room or at night), the iris dilator muscle (which runs radialy through the iris) pulls away from the centre, causing the pupil to ‘dilate’. This allows more light to reach to the retina. When too much light is entering the eye, the iris sphincter muscle (which enriches the pupil) pulls toward the centre, causes the pupil to constrict, allowing less light to reach the retina