The pupil of the eye is really a circular muscle.

Question

Quizplus · Accepted Answer

The pupil is an opening in the center of the iris, which allows light to enter the eye. It is not a muscle, but it is controlled by the circular and radial muscles of the iris. The circular muscle, also known as the sphincter pupillae muscle, contracts to constrict the pupil and reduce the amount of light that enters the eye, while the radial muscle, known as the dilator pupillae muscle, relaxes to widen the pupil and let in more light.

The Pupil of the Eye Is Really a Circular Muscle