Which muscle completely surrounds the eye socket and closes the eyelid?

The muscle that completely surrounds the eye socket and is responsible for closing the eyelid is the orbicularis oculi. This muscle functions as a circular band of muscle fibers that encircle the eye, enabling actions such as blinking and squinting. When the orbicularis oculi contracts, it draws the eyelids together, creating a tight closure over the eye. This is essential for protecting the eye from irritants, preventing excessive exposure to light, and aiding in the moisturization of the eye surface.

The other muscles listed do not serve this specific function. The frontalis muscle primarily elevates the eyebrows and wrinkles the forehead, the orbicularis oris is involved in movements of the lips, such as puckering, and the zygomaticus muscle facilitates smiling by raising the corners of the mouth. Each of these muscles has distinct roles in facial expression, but none encircle the eye or operate to close the eyelid like the orbicularis oculi does.