The eyelid is located at the front of the orbit and covers the surface of the eye. It is divided into the upper and lower lids, the free edge of which is called the lid margin, and the fissure between the upper and lower lid margins, called the lid fissure, whose inner and outer junctions are called the inner and outer canthus, respectively. The upper lid covers the upper cornea by 1-2 mm, and the inner canthus has a small fleshy elevation called the lacrimal fossa, which is a metaplastic skin tissue. The eyelid is divided into five layers from outside to inside: the skin layer, the subcutaneous tissue layer, the muscle layer, the lid layer, and the conjunctiva layer. The normal position of the eyelid is as follows: i. The eyelid is closely attached to the surface of the eye with a potential capillary gap in between; ii. The upper and lower eyelashes should be fully extended pointing forward, neatly aligned, not in contact with the cornea, and able to block dust, sweat, etc. from invading the eye; iii. The upper and lower eyelids can close tightly; iv. The upper lid can be lifted up to the upper edge of the pupil; and v. The upper and lower tear dots are attached to the base of the lacrimal mound so that tears can enter the tear duct.