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

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“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, LondonMay 22, 1983
Sally Foy

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