Eyelid

eyelid, movable tissue, consisting primarily of skin and muscle, that shields and protects the eyeball from mechanical injury and helps to provide the moist chamber essential for the normal functioning of the conjunctiva and cornea. The conjunctiva is the mucous membrane that lines the eyelid and covers the visible portion of the eyeball except the cornea (the transparent part of the eyeball that covers the iris and the pupil). Each eyelid contains a fibrous plate, called a tarsus, that gives it structure and shape; muscles, which move the eyelids; and meibomian (or tarsal) glands, which secrete lubricating fluids. The lids are covered with skin, lined with mucous membrane, and bordered with a fringe of hairs, the eyelashes. The lids move through the action of a circular lid-closing muscle, the orbicularis oculi, and of the lid-raising muscle, the levator of the upper lid. Impulses for closing come by way of the facial (seventh cranial) nerve, and for opening by way of the oculomotor (third cranial) nerve. The lid borders are kept lubricated by an oily secretion (called sebum) of the meibomian glands. This secretion forms part of the tear film and reduces evaporative tear loss.