Which part of the eye controls pupil size?

The iris is the part of the eye that controls pupil size, playing a crucial role in regulating the amount of light that enters the eye. It is a thin, circular structure located behind the cornea and in front of the lens, and it contains muscles that can contract or relax.

When the light is bright, the muscles of the iris contract, causing the pupil to constrict and limit the amount of light that reaches the retina, thereby protecting it from excessive brightness. Conversely, in low light conditions, the iris muscles relax, leading to an expansion of the pupil, which allows more light to enter and enables better vision in dim environments.

Understanding the iris's function is vital in explaining how our visual system adapts to varying lighting conditions, contributing to our ability to see clearly in different environments. The pupil itself does not have any muscle tissue; it is merely the opening that changes size due to the actions of the iris. The lens and fovea serve different functions related to focusing light and providing sharp central vision, respectively.