An eyelid is a thin fold of skin and muscle that covers and protects an eye. 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 eyelashes which serve to heighten the protection of the eye from dust and foreign debris. "Palpebral" means relating to the eyelids.

Anatomy

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Innervation

In humans, the ophthalmic branch of the trigeminal nerve provides sensation to the upper eyelid and the maxillary branch of the trigeminal nerve provides sensation to the lower lid.

Usage

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When an eye becomes dry, closing the eyelid and opening it again rapidly — referred to as "blinking" — 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 "winking."

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 is autonomic). Without eyelids, many animals would be helpless to block visual sensory overload under extremely bright light. Many animals also use eyelids to block light from reaching the eyes during sleeping cycles.

Types

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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. Outer eyelids in most animals move vertically.

Many terrestrial vertebrates 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 as well as serve as a barrier while swimming to land animals. Camels, Crocodiles, birds, and polar bears are among the others which have this membrane. The nictitating membrane sweeps across the eye diagonally or horizontally. In humans, the 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.

See also

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Eyelid disorders

• Blepharospasm
• Cellulitis
• Chalazion
• Dermatochalasis
• Ectropion
• Entropion
• Ptosis
• Stye (Hordeolum)

Eyelid surgeries

• Asian blepharoplasty
• Blepharoplasty
• Epicanthoplasty

External links

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• Eyelid Anatomy

Eye

Augenlid | Párpado | Paupière | Palpebra | Vokas | Ooglid | まぶた | Powieka | Pálpebra | Ögonlock