- The anatomical structure in the eye that regulates the amount of light passing through the pupil is the iris.
- The iris is a pigmented, muscular diaphragm located between the cornea and the lens.
- It controls the size of the pupil by contracting or dilating its muscles, thereby adjusting the amount of light that enters the eye.

- The cornea is the transparent outer layer that helps focus light but does not regulate light entry.
- The lens further focuses light onto the retina but does not control light intensity.
- The retina is the light-sensitive layer at the back of the eye where visual signals are processed, but it doesn't regulate light entry.

Thus, the iris functions much like the aperture of a camera, making sure that the right amount of light reaches the retina for optimal vision under different lighting conditions.

Reference: Snell's Clinical Anatomy, Volume 1, Chapter 5, Page 124