   #copyright

Eyelid

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

   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

Layers

   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 meibomium glands lie
   within the eyelid and secrete tears. They often become blocked leading
   the blepharitis

Innervation

   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) of the trigeminal
   nerve.

Blood supply

   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.

Usage

   Blinking eye
   Blinking eye

   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

   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 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.

   Retrieved from " http://en.wikipedia.org/wiki/Eyelid"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
