What anatomical structures serve as the first line of defense for the eyes, causing them to blink when touched?

The correct answer is based on the function of the cilia, also known as eyelashes, which serve as the first line of defense for the eyes. When the cilia are touched, they trigger a reflex that causes the eyelids to blink, effectively protecting the eyes from potential irritants or foreign objects. This blink reflex serves as a quick response to stimuli, helping to keep the surface of the eye moist and clear of debris.

The cornea is the transparent layer covering the front of the eye and is highly sensitive, but it is not primarily responsible for the blinking action. The conjunctiva is a membrane that covers the front of the eye and inner eyelids, playing a role in lubricating and protecting the eye but is not directly responsible for initiating the blink; rather, it is involved in the broader function of eye protection. Eyebrows help keep sweat and debris out of the eyes but do not play a direct role in the reflexive blinking response.

Understanding these roles helps clarify why the cilia, or eyelashes, are correctly identified as the anatomical structures that prompt the blink reflex when touched.