The muscle that helps to control the eyelid is called the orbicularis oculi. Closing the eyelid can either be voluntary or involuntary, and this action primarily keeps the eyeball moist and protects it from foreign objects. There are a couple conditions that can affect the eyelid muscle, but these are generally harmless.
Usually, the eyelid muscle's action to close the eyelid is involuntary. On average, a person uses the eyelid muscles to blink about every five or six seconds. Although many people don't think about it much, blinking serves a couple very important purposes.
The act of the eyelid closing spreads moisture across the surface of the eyeball. Tears produced by the tear ducts and fatty secretions produced by the Meibomian glands are wiped along the eyeball whenever a person blinks. These secretions help keep the cornea and other parts of the eyeball moist. They also help to wash away any dust or other foreign particles from the eyeball.
Using an eyelid muscle to blink helps to keep foreign particles off the eyeball. When the eyelid is blinked, the eyelashes are lowered and they act as a type of screen, keeping out the majority of dirt, dust, and tiny bugs. If something gets on the eyeball, tear production usually increases, along with the speed at which a person blinks. Increased tear production, along with the rapid blinking action, helps to wash and sweep out any tiny dust particles that could potentially harm the sensitive tissue of the eyeball.
Bright lights cause people to shut their eyes. By doing this, a person can possibly save his sight, since some extremely bright, intense lights can harm the eyeball. For example, staring at the sun for too long can possibly kill the cells in the retina of the eye.
Blepharospasm is the scientific term for eyelid twitching. This happens when the eyelid muscle starts to spasm, or contract involuntarily, which causes the eyelid to twitch. Although it can be quite annoying, this is usually not harmful, and it is often caused by lack of sleep, excessive caffeine, or some medications. This can also be a withdrawal symptom of certain drugs, and in rare cases, it can be caused by a concussion or damage to the basil ganglia in the brain.
Ptosis is a medical condition that causes the eyelid muscle to droop. This condition can afflict one or both eyes, and often occurs as people get older. Some neurological conditions, certain types of trauma or eye surgeries, and birth defects can also cause the eyelid to become weak or damaged.
Many times, no treatment is necessary for ptosis. If the condition is severe enough, however, surgery may be required, but usually only if the eyelid is in the way of the pupil, affecting a person's vision. The surgeon usually tightens the eyelid muscle, which, in turn, causes the eyelid to raise.