Pupillary Muscles: Sphincter & Dilator Explained - Eye Anatomy

Sphincter pupilae: This is a circular muscle that controls the size of the pupil. When the sphincter pupillae contracts, the pupil gets smaller. When it relaxes, the pupil gets bigger.

Dilator pupillae: This is a radial muscle that controls the size of the pupil. When the dilator pupillae contracts, the pupil gets bigger. When it relaxes, the pupil gets smaller.