Which structure controls pupil diameter in response to light?

Which structure controls pupil diameter in response to light?

• A. The retina
• B. The cornea
• C. The iris
• D. The sclera

Answer: (C)

The pupil size is controlled by the iris, which has two sets of muscles that adjust the diameter in response to light. When there is bright light, the circular muscles of the iris contract, making the pupil smaller to limit light entering the eye. In dim light, the radial muscles contract, widening the pupil to let in more light. The retina detects light levels and sends signals that trigger this reflex, but it is the iris that actually changes the pupil diameter. The cornea and sclera are structural parts of the eye and do not regulate pupil size.

The pupil size is controlled by the iris, which has two sets of muscles that adjust the diameter in response to light. When there is bright light, the circular muscles of the iris contract, making the pupil smaller to limit light entering the eye. In dim light, the radial muscles contract, widening the pupil to let in more light. The retina detects light levels and sends signals that trigger this reflex, but it is the iris that actually changes the pupil diameter. The cornea and sclera are structural parts of the eye and do not regulate pupil size.