The iris sphincter muscle is a circular band of smooth muscle located in the iris of the eye. Its primary function is to control the size of the pupil, which regulates the amount of light entering the eye. When the muscle contracts, the pupil constricts, allowing less light in, which is important in bright conditions. This muscle is innervated by the parasympathetic nervous system, specifically through the oculomotor nerve. The iris sphincter works in conjunction with the dilator pupillae muscle, which opens the pupil in low-light conditions. Together, they help maintain optimal vision in varying light environments.