Kobi Russell of Palmerston North asks :-

Why do our eye lids close vertically not sideways?

Christine Thomson, a veterinary anatomist at Massey University, responded.

Eyelids have evolved to protect and lubricate the eye. The eyes of aquatic animals are constantly bathed in water, thus many species don’t have eyelids or tear glands. Some aquatic animals do have eyelids to protect the eyes during fighting, or for bottom dwellers such as flounder, to protect the eyes from sand. Eyelids may provide protection against bright light, too.

Conversely, most terrestrial vertebrates need eyelids and tear glands because they live in a dry environment. Eyelid movement spreads moisture (tears) across the cornea at the front of the eye, and also protects the eye from dust and strong light.

Some animals, such as geckos, have a transparent third eyelid permanently covering the cornea. But the eyelids on most terrestrial animals are mobile and close by moving vertically. The centre portions are freely movable, but the edges are anchored by small, palpebral ligaments. Thus, when eyelids close they produce a horizontal slit. The central portions of the lids can be reasonably thick and in the upper eyelid form a tissue ridge that overhangs and protects the eyes. The protective function of the upper eyelid is greatly enhanced by the presence of eyelashes that prevent downward falling particles from getting into the eye. At the corners of the eye, where the eyelids are anchored, the lids recede and blend with surrounding skin.

If eyelids closed by moving horizontally, the non-protuberant, fixed point of the eyelids would be above the eye. There would be no movable ridge to overhang and protect the eye. Similarly, horizontally-moving eyelids would not have the movable dust collector (eyelashes) protecting the eye from falling debris. No mention of horizontally moving eyelids was found on the scientific data bases. Thus, vertically moving eyelids are probably a successful evolutionary adaption to protect the eyes of animals and humans.