An 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
Eyelash es which serve to heighten the protection of the eye from dust and foreign debris. "Palpebral" (and "blepharo") means relating to the eyelids.
In the human eyelid, there are various layers; from superficial to deep, they are:
Skin ,
Subcutaneous Tissue ,
Orbicularis Oculi ,
Orbital Septum & tarsal plates, palpebral
Conjunctiva . The meibomian glands lie within the eyelid and secrete tears. They often become blocked leading the blepharitis.
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.
In humans, the eyelids are supplied by 2 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.
When an eye becomes dry, closing the eyelid and opening it again rapidly — referred to as "
Blink ing" — 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 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.
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 as well as serve as a barrier while swimming to land animals. Camels, crocodiles, cats, 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.
A double eyelid is a type of eyelid where a crease exists. The opposite of a double eyelid is a single eyelid, or an eyelid without a crease. There is a wide variation in the location of the crease, particularly in the Asian double eyelid. Further discussion and pictures of the variations in Asian double eyelids can be found
here . A double eyelid can be created in a person with a single eyelid by
Asian Blepharoplasty or "Double Eyelid Surgery" through a variety of methods that depend on the anatomy of the eyelid and the desired appearance.http://www.MirandaPlasticSurgery.com/asian_eyelids.html
.]]
