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skin folds around the eyes in vertebrate animals and man. Eyelids serve to protect the eyeball from external injury, to moisten it with lachrymal fluid, and to remove foreign bodies from the eye. The majority of animals have paired (upper and lower), movable eyelids. In a number of sharks, amphibians, reptiles, birds, and some mammals, there is also a third movable eyelid—the nictitating membrane. In most mammals the third eyelid lacks musculature and is therefore immovable; in primates and in man it is reduced and forms a semilunar fold in the inner corner of the eye.
Eyelids in man consist of upper and lower movable skin folds that cover the eyeball when closed and circumscribe the eye aperture. The boundary of the upper lid is the recess between the orbit and the eyeball; the lower lid is separated from the cheek by a poorly defined furrow. The eyelids consist of skin, subcutaneous tissue, a muscle layer, a cartilaginous plate, and the conjunctiva. The skin of the eyelids is thin and turns into conjunctiva along the edges, where the eyelashes grow and the openings of the glands are located. The blood vessel network of the eyelids is well developed; sensory innervation is effected by branches of the trigeminal nerve; motor innervation is by the facial nerve.
Of greatest anthropological interest is the upper lid, the skin of which can form various types of folds—superior (suprapalpebral), median (palpebral), and inferior (tarsal). The last type of fold is more developed in the outer part of the eye and is manifested more strongly with age. Also distinguished is the Mongolian fold (epicanthic), which partially covers the lacrimal caruncle and is usually a continuation of the palpebral fold. The epicanthic fold is especially widespread among representatives of the Mongoloid race (up to 60 percent or more, especially frequent in women and children); it is absent among Caucasoids and Negroids (except for Bushmen).