Eyelid
|
|
An eyelid is a thin membrane of skin with the purpose of covering and protecting an eye. A muscle is used to slide the membrane over the surface of the eye ("closing" the eye) and to retract the membrane ("opening" the eye). This can be either voluntarily or involuntarily. The human eyelid feature a row of eyelashes which serve to heighten the protection of the eye from dust and foreign debris.
| Contents |
Usage
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 an animal (usually human) 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, and a common children's game is to see who will not be able to maintain control over the reflex.
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
Humans have two eyelids per eye in the form of an upper and lower lid. Each eyelid has a range allowing it to cover one half of the exposed eye surface. Humans cannot control the upper and lower eyelids independently; they must operate as a pair. Outer eyelids in most animals move vertically.
Many terrestrial vertebrates have a 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 nictitating membrane is the apparently useless pink lump in the inner corner of the eyes.
Fish do not have eyelids. It has been suggested that eyelids evolved as a way to clean out the eyes. As fish have a constant stream of water flowing over their eyes, they do not need specialized membranes to perform this function.