Eyelid

Eyelid
Closed human eye showing eyelid and a set of lashes which grow along the edge of the lid
The same eye with its eyelid pulled open, displaying meibomian glands on the inside and a set of lashes growing in multiple layers
Details
System	Integumentary
Function	Covers and protects the eye by blinking or closing, keeping the cornea moist
Identifiers
Latin	palpebrae
MeSH	D005143
TA98	A15.2.07.024
TA2	114, 115
FMA	54437
Anatomical terminology [edit on Wikidata]

This article about biology may be excessively human-centric. Please improve coverage for other species and discuss this issue on the talk page. (June 2019) (Learn how and when to remove this message)

An eyelid (/ˈaɪ.lɪd/ EYE-lid) 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. "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. A set of specialized hairs known as lashes grow from the upper and lower eyelid margins to further protect the eye from dust and debris.

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.