What is the colored portion of the eye that regulates the size of the pupil?

The colored portion of the eye that regulates the size of the pupil is the iris. The iris is a thin, circular structure that is situated in front of the lens and behind the cornea. It consists of muscle fibers that can expand and contract, allowing it to control the amount of light entering the eye through the pupil. This function is crucial for protecting the retina and helping the eye to adjust to different lighting conditions, enhancing vision clarity.

While the cornea is responsible for focusing light onto the retina and the lens further fine-tunes that focus, they do not have the ability to change size or regulate pupil diameter. The sclera, often referred to as the "white" of the eye, serves as a protective outer layer, but it also lacks any role in adjusting light intake. Thus, the iris is uniquely responsible for controlling pupil size and, by extension, light entry into the eye.