What muscle regulates the amount of light entering the eye?

The iris is the part of the eye that regulates the amount of light entering by controlling the size of the pupil. It is a muscular structure that can expand or contract, adjusting the diameter of the pupil in response to varying lighting conditions. In bright light, the iris contracts to reduce the size of the pupil, limiting the light that enters the eye and protecting the delicate internal structures; in dim light, the iris relaxes, causing the pupil to dilate and allow more light to enter, enhancing visibility. The ability of the iris to perform this function is crucial for optimal vision under changing environmental lighting.