Nictitating membrane
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Many species of land animals have a nictitating membrane, which can move across the eyeball to give the sensitive eye structures additional protection in particular circumstances. It is often called a third eyelid or haw and may be referred to as the plica semilunaris in more advanced applications.
Nictitating membranes are found in crocodiles, lizards, birds, frogs, most species of sharks, and some species of mammals. In humans, the nictitating membrane is the apparently useless pink lump in the inner corner of the eyes; it is permanently folded into that corner and no longer functions, apparently rendered redundant by evolution some time in the past. In fact, only one species of primate -- the Calabar angwantibo -- is known to have a functioning nictitating membrane.
Unlike human eyelids, the nictitating membrane moves horizontally across the eyeball. It is normally translucent. In some diving animals, for example beavers and manatees, it moves across the eye to protect it while under water, and in these species it is transparent; in other diving animals including sea lions, it is activated on land, to remove sand and other debris. This is its function in most animals. In birds of prey, it also serves to protect the parents' eyes from their chicks while they are feeding them. In polar bears it protects the eyes from snow blindness. In sharks it protects the eye while the shark strikes at its prey. Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree (or whatever it is that they are pecking) in order to prevent their eyes from leaving their sockets.
In cats and dogs, the nictitating membrane is not usually visible, and its being chronically visible should be taken as a sign of poor condition or ill health.
In many species, any stimulus to the eyeball (such as a puff of air) will result in reflex nictitating membrane response. This reflex is widely used as the basis for experiments on classical conditioning in rabbits.
