Iris

The coloured part of the eye, made up of a loose framework of transparent collagen and muscle fibres, that lies behind the cornea and in front of the lens. It is connected at its outer edge to the ciliary body and has a central aperture, the pupil, through which light enters the eye and falls on the retina. The iris constantly contracts and dilates to alter the size of the pupil, which controls the amount of light that passes through the pupil.