Which part of the eye is responsible for controlling the size of the pupil?

The iris is the part of the eye responsible for controlling the size of the pupil. It is a thin, circular structure located in front of the lens and is composed of muscle fibers that can contract or relax. When light enters the eye, the iris adjusts the size of the pupil to regulate the amount of light that reaches the retina. In bright light conditions, the muscles in the iris contract, causing the pupil to become smaller, which helps prevent excessive light from damaging the sensitive retina. In dim lighting, the iris relaxes, and the pupil enlarges to allow more light to enter for better vision.

The other parts mentioned do not play a direct role in controlling pupil size. The retina is responsible for converting light into neural signals, which are then sent to the brain for visual processing. The ciliary body is involved in the production of aqueous humor and the accommodation of the lens for focusing but does not manage pupil size. The cornea is primarily responsible for the initial refraction of light as it enters the eye but also does not control the pupil.