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The eyelids (palpebræ) are two thin, movable folds, placed in front of the eye, protecting it from injury by their closure.
The upper eyelid is the larger, and the more movable of the two, and is furnished with an elevator muscle, the Levator palpebræ superioris.
When the eyelids are open, an elliptical space, the palpebral fissure (rima palpebrarum), is left between their margins, the angles of which correspond to the junctions of the upper and lower eyelids, and are called the palpebral commissures or canthi.
The lateral palpebral commissure (commissura palpebrarum lateralis; external canthus) is more acute than the medial, and the eyelids here lie in close contact with the bulb of the eye: but the medial palpebral commissure (commissura palpebrarum medialis; internal canthus) is prolonged for a short distance toward the nose, and the two eyelids are separated by a triangular space, the lacus lacrimalis. At the basal angles of the lacus lacrimalis, on the margin of each eyelid, is a small conical elevation, the lacrimal papilla, the apex of which is pierced by a small orifice, the punctum lacrimale, the commencement of the lacrimal duct.
The eyelids are composed of the following structures taken in their order from without inward: integument, areolar tissue, fibers of the Orbicularis oculi, tarsus, orbital septum, tarsal glands and conjunctiva. The upper eyelid has, in addition, the aponeurosis of the Levator palpebræ superioris