Which of the following regulates the amount of light entering the eye?

The iris muscles regulate the amount of light entering the eye by altering the diameter of the pupil. Autonomic reflexes regulate pupil diameter in response to light levels. When bright light stimulates the eye, parasympathetic stimulation causes the constriction of the pupil by stimulating the contraction of the circular muscles. In dim light, sympathetic stimulation causes the dilation of the pupil by stimulating the contraction of the radial muscles.