View source for Eyelid

{{Infobox Anatomy |
  Name         = {{PAGENAME}} |
  Latin        = palpebra inferior, palpebra superior |
  GraySubject  = 227 |
  GrayPage     = 1025 |
  Image        = eye makeup.jpg|
  Caption      = Upper and lower eyelids|
  Image2       = |
  Caption2     = |
  Precursor    = |
  System       = |
  Artery       = [[lacrimal artery|lacrimal]], [[superior palpebral artery|superior palpebral]], [[inferior palpebral artery|inferior palpebral]] |
  Vein         = |
  Nerve        =  ''upper:'' [[Infratrochlear nerve|infratrochlear]], [[Supratrochlear nerve|supratrochlear]], [[Supraorbital nerve|supraorbital]], [[lacrimal nerve|lacrimal]]<BR>''lower:'' [[Infratrochlear nerve|infratrochlear]], branches of [[infraorbital nerve|infraorbital]] |
  Lymph        = |
  MeshName     = Eyelids |
  MeshNumber   = A01.456.505.420.504 |
  DorlandsPre  = |
  DorlandsSuf  = |
}}
{{CMG}}
__NOTOC__


==Overview==

An '''eyelid''' is a thin fold of skin that covers and protects an [[eye]]. With the exception of the [[prepuce]] and the [[labia minora]], it has the thinnest skin of the whole body. The [[levator palpebrae superioris muscle]] retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily.  The human eyelid features a row of [[eyelash]]es which serve to heighten the protection of the eye from dust and foreign debris. "Palpebral" (and "blepharo") means relating to the eyelids.

==Anatomy==
===Layers===
In the human eyelid, there are various layers; from superficial to deep, they are:
[[skin]], [[Subcutis|subcutaneous tissue]], [[Orbicularis oculi muscle|orbicularis oculi]], [[orbital septum]] & [[Tarsus (eyelids) | tarsal plates]], and palpebral [[conjunctiva]]. The meibomian glands lie within the eyelid and secrete the lipid part of the [[tears | tearfilm]].

====Skin====
The skin is similar to areas elsewhere, but have 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.  <ref> "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 5 Apr. 2008 </ref>

===Innervation===
In humans, the sensory nerve supply to the upper eyelids is from the [[infratrochlear]], [[supratrochlear]], [[supraorbital]] and the [[lacrimal nerve]]s 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) of the trigeminal nerve. 

===Blood supply===
In humans, the eyelids are supplied by two arches on each upper and lower lid. The arches are formed by anastamoses of the [[lateral palpebral arteries]] and [[medial palpebral arteries]], branching off from the [[lacrimal artery]] and [[ophthalmic artery]], respectively.

===Muscles besides the Orbicularis oculi===
Involuntary muscle fibers controlled by the sympathetic autonomic nervous system and open the eyelids away from each other, widening the eye aperture.  The [[corrugator supercilii]] helps shield the eye from sun glare by pulling the eyebrow toward the bridge of the nose making a "roof" over the medial side of the eye and typical furrows in the forehead.  The [[procerus]] muscle, on the bridge of the nose, fold the skin into tranmsverse furrows.  The [[Frontalis]]
lifts the eyebrows (thus opposing the orbital portion of the orbicularis), especially when looking up. It also acts when a view is too distant or dim.<ref> Ibid </ref>

==Usage==
[[Image:Blinking.gif|frame|left|Blinking eye]]When an eye becomes dry, "[[blink]]ing" – closing the eyelid and opening it again rapidly – can help to spread moisture across the surface of the eye and ease the discomfort.  Blinking also serves the purpose of helping to remove irritants which have landed in the eye.  When a person chooses to blink one eye as a signal to another in a social setting, it is known as "[[wink]]ing."

Most animals with eyelids have a [[reflex]] to close the eyes when a threat comes too near.  This is done involuntarily to protect the eye from contact with the threat.  It is often strong enough to overcome any voluntary resistance.

Eyelids also serve the purpose of helping the animal to control the amount of light entering the eye (control of the [[Iris (anatomy)|iris]] is autonomic).  Without eyelids, many animals would be helpless to block visual sensory overload under very bright light.  Many animals also use eyelids to block light from reaching the eyes during sleeping cycles.

==Types==
In humans, each eye has an upper and lower eyelid which operate as a pair, however, it is primarily the upper eyelid that moves across the exposed surface of the eye during blinking. Lower eyelids in most animals move vertically.

Many terrestrial [[vertebrate]]s have an eyelid known as a [[nictitating membrane]], or ''haw''. This  eyelid is closer to the eye than the outer lids and is usually transparent.  The purpose of the nictitating membrane is to add protection to the eye from debris and irritation, and in land animals it also serves as a barrier while swimming.  Camels, crocodiles, cats, birds, and polar bears are among animals that have this membrane.  The nictitating membrane sweeps across the eye diagonally or horizontally. In humans, the [[Plica_semilunaris_of_the_conjunctiva|plica semilunaris]] (or ''semilunar fold'') is thought to be the [[vestigial]] remnant of the nictitating membrane.

It has been suggested that eyelids evolved as a way to remove debris from the eyes. Given that fish have a constant stream of water flowing over their eyes, it is not surprising that they do not have eyelids or need specialized membranes to perform this function.

===Double eyelid===
A double eyelid is a type of eyelid where a crease exists. In contrast, a single eyelid has no crease. There is a wide variation in the location of the crease, particularly in the Asian double eyelid.

==Death==
After [[death]], it is common in many cultures to pull the eyelids of the [[deceased]] down to close the eyes. This a typical part of the [[last offices]].

==See also==
===Disorders===
[[Image:Stye02.jpg|thumb|right|300px|Eyelid affected by [[Stye]].]]
*[[Blepharospasm]]
*[[Cellulitis]]
*[[Chalazion]]
*[[Dermatochalasis]]
*[[Ectropion]]
*[[Entropion]]
*[[Hay-Wells syndrome]]
*[[Ptosis (eyelid)|Ptosis]]
*[[Stye|Stye (Hordeolum)]]

===Surgeries===
*[[Asian blepharoplasty]]
*[[Blepharoplasty]]
*[[Epicanthoplasty]]

==Additional images==
<gallery>
 Image:Eye makeup.jpg|Eye makeup
 Image:Gray515.png|Bloodvessels of the eyelids, front view.
 Image:Gray866.png|Horizontal section through the eye of an eighteen days’ embryo rabbit. X 30.
 Image:Gray888.png|Sagittal section of right orbital cavity.
 Image:Gray893.png|Sagittal section through the upper eyelid. 
 Image:Gray894.png|The tarsi and their ligaments. Right eye; front view.
 Image:Gray896.png|The lacrimal apparatus. Right side.
</gallery>

==References==
<references/>

<br>

{{Accessory organs of the eye}}

[[Category:Eye]]

[[bn:পলক (চোখ)]]
[[bg:Клепач]]
[[de:Augenlid]]
[[es:Párpado]]
[[eo:Palpebro]]
[[eu:Betazal]]
[[fr:Paupière]]
[[id:Kelopak mata]]
[[it:Palpebra]]
[[he:עפעף]]
[[la:Palpebra]]
[[lt:Vokas]]
[[nl:Ooglid]]
[[ja:まぶた]]
[[pl:Powieka]]
[[pt:Pálpebra]]
[[ru:Веко]]
[[fi:Silmäluomi]]
[[sv:Ögonlock]]
[[yi:אויגן לאד]]

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