An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eyelid features a row of eyelashes along the eyelid margin, which serve to heighten the protection of the eye from dust and foreign debris, as well as from perspiration. "Palpebral" (and "blepharal") means relating to the eyelids. Its key function is to regularly spread the tears and other secretions on the eye surface to keep it moist, since the cornea must be continuously moist. They keep the eyes from drying out when asleep. Moreover, the blink reflex protects the eye from foreign bodies. The appearance of the human upper eyelid often varies between different populations. The prevalence of an epicanthic fold covering the inner corner of the eye account for the majority of East Asian and Southeast Asian populations, and is also found in varying degrees among other populations. Separately, but also similarly varying between populations, the crease of the remainder of the eyelid may form either a "single eyelid", a "double eyelid", or an intermediate form. Eyelids can be found in other animals, some of which may have a third eyelid, or nictitating membrane. A vestige of this in humans survives as the plica semilunaris. Structure Layers The eyelid is made up of several layers; from superficial to deep, these are: skin, subcutaneous tissue, orbicularis oculi, orbital septum and tarsal plates, and palpebral conjunctiva. The meibomian glands lie within the eyelid and secrete the lipid part of the tear film. Skin The skin is similar to areas elsewhere, but is relatively thin and has more pigment cells. In diseased persons these may wander and cause a discoloration of the lids. It contains sweat glands and hairs, the latter becoming eyelashes as the border of the eyelid is met. The skin of the eyelid contains the greatest concentration of sebaceous glands found anywhere in the body. Nerve supply In humans, the sensory nerve supply to the upper eyelids is from the infratrochlear, supratrochlear, supraorbital and the lacrimal nerves from the ophthalmic branch (V1) of the trigeminal nerve (CN V). The skin of the lower eyelid is supplied by branches of the infratrochlear at the medial angle. The rest is supplied by branches of the infraorbital nerve of the maxillary branch (V2)