Which structure assists in the distribution of tear film over the corneal surface and limits light entering the pupil?

The structure that assists in the distribution of tear film over the corneal surface and limits light entering the pupil is the upper and lower eyelids. The eyelids play a crucial role in maintaining the health of the eye by providing coverage and protection. When the eyelids close, they help spread the tear film evenly across the cornea, ensuring that it remains lubricated and clear.

Additionally, the eyelids function as a barrier, regulating the amount of light that enters the eye. By blinking, they not only distribute tears but also can limit excess light exposure, protecting sensitive retinal cells from overexposure that can lead to discomfort or damage. This dual functionality makes the eyelids essential for both the mechanical and protective aspects of vision.

The cornea is primarily involved in light refraction and is not actively involved in the distribution of tear film. The sclera, while providing structural support, does not play a role in tear film distribution or light control. The iris, which controls the diameter and size of the pupil, regulates light entry but does not assist in spreading the tear film over the cornea. Thus, the upper and lower eyelids are correctly identified as the structures responsible for these functions.