What forms the first line of defense in the protective system of the eye?

The palpebrae, or eyelids, serve as the first line of defense in the protective system of the eye for several reasons. They physically shield the eye from external hazards such as dust, debris, and potential trauma. When an object approaches the eye, the eyelids will reflexively close, providing immediate protection. Additionally, the eyelids help to moisten the eye surface by spreading tears and preventing desiccation.

While the cornea is crucial for vision and acts as a barrier with its epithelium, it does not provide the same physical protective barrier that the eyelids do. The iris, known for its role in controlling the amount of light entering the eye, does not contribute to external protection. Lachrymal glands are important for producing tears to maintain lubrication and remove irritants but are not a barrier against physical threats. Thus, the palpebrae are essential for the initial mechanical defense of the eye.