The iris is part of the uveal tract. It works to vary the size of the pupil and therefore controls the amount of light which enters the eye. To do this, the iris has two sets of muscles;
• Sphincter muscle, which encircles the pupil and constricts the pupil, primarily through parasympathetic innervation.
• The dilator muscle, whose radially orientated fibres allow for pupil dilation, predominantly through sympathetic innervation.
