The dilator pupillae muscle is a smooth 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 causes the pupil to dilate, or widen, allowing more light to pass through. This muscle is innervated by the sympathetic nervous system, which is responsible for the body's "fight or flight" response. The dilation of the pupil helps improve vision in low-light conditions and is an important part of the eye's ability to adapt to varying light levels.