Iris

The iris is the anterior-most part of the uvea (middle layer of the eye) and controls the amount of light entering the eye. The opening and closing action of the iris is performed by two muscles that are innervated by the parasympathetic nervous system: the musculus sphincter pupillae that encircles the pupil border and causes a reduction in pupil size, and the musculus dilatator pupillae whose fibres run in a radial pattern within the iris and causes an increase in pupil size. In terms of anatomy, the iris separates the anterior chamber of the eye from the posterior chamber. Its pigmentation can differ significantly and determines the colour of the eye.