Which muscle is responsible for the primary action of eyelid closure?

The primary muscle responsible for eyelid closure is the orbicularis oculi. This muscle encircles the eye and allows for the rapid and deliberate closing of the eyelids, which is essential for protecting the eye from environmental factors and for maintaining moisture. The orbicularis oculi is involved in both blinking and squinting, enabling quick movements that are necessary for these actions.

In contrast, the levator palpebrae is responsible for elevating the upper eyelid, facilitating opening rather than closure. The muscle of Horner primarily refers to the smooth muscle associated with the eyelid, assisting in fine-tuning eyelid closure, but it is not the main player in this action. The frontalis muscle is involved in raising the eyebrows and does not contribute to eyelid closure at all. Therefore, the orbicularis oculi is correctly identified as the muscle that predominantly performs the essential function of closing the eyelids.