The Sphincter Pupillae Muscle is a small, circular muscle located in the iris of the eye. Its primary function is to control the size of the pupil, which is the opening that allows light to enter the eye. When this muscle contracts, it constricts the pupil, reducing the amount of light that reaches the retina, which helps protect the eye in bright conditions. This muscle is innervated by the parasympathetic nervous system, specifically through the oculomotor nerve. The Sphincter Pupillae works in conjunction with the dilator pupillae muscle, which enlarges the pupil in low-light conditions. Together, they help regulate vision by adjusting the amount of light entering the eye.