The eyes of mammals are protected from dirt and impacts by eyelids.


"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)

The grand design: Form and colour in animalsBLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, LondonOctober 21, 2016
Sally Foy