The iris is composed of muscle layers - an outer layer of longitudinal smooth muscle and an inner layer of circular smooth muscle. The inner circular layer is innervated by the parasympathetic nerves. When these muscles contract, they that cause the pupil to constrict in bright light. the The outer layer of longitudinal smooth muscle is innervated by the sympathetic nerves. When these muscles contract, they that cause the pupil to dilate in dim light.
