The eyelids of mammals provide lubrication for the eye using teardrops that are applied during blinking.
“Being a particularly delicate instrument, the eye needs protection — usually, an eyelid. Most mammals have two eyelids, one above and one below, but some – such as horses and deer – have a third, inner eyelid, the nictitating membrane, which may move upwards or sideways across the eyeball. Both types of eyelid can be closed to protect the eye from a blow, or from dirt; in closing – blinking – they wipe the eyeball clean and lubricate it with teardrops.” (Foy and Oxford Scientific Films 1982:124)
Foy S, Oxford Scientific Films. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J. M. Dent & Sons Ltd, London; 1982. 238 p.
