The eyelids play a key role in protecting the eyes. They sweep away debris when the eyes close and help spread moisture (tears) over the surface of the eyes when they open. The eyelids provide a mechanical barrier against injury by closing rapidly when needed.
Where Tears Come From
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Tears are produced by the main and accessory lacrimal glands and collected in ducts at the inner corner of each eye. From there, they drain into the nose. The main lacrimal gland is responsible for reflex tearing, which occurs, for example, when the eye gets irritated by a foreign body or from cutting an onion. Accessory lacrimal glands are responsible for keeping the eye well lubricated, comfortable, and free of dust, all of which help with clear vision.
An abnormality of the tear (lacrimal) glands can lead to insufficient tear volume or to an abnormality in the composition of the tears themselves. Without adequate tear volume or normal tear composition, the eyes can dry, be more vulnerable to irritation from vapors and airborne particles, and may be unable to normally fight infections. Abnormal tear production may be due to a problem within the tear glands and ducts (lacrimal excretory ducts, which carry tears into the eye) or due to a bodywide (systemic) disease that affects the tear glands, such as Sjögren syndrome Sjögren Syndrome Sjögren syndrome is a common autoimmune connective tissue disorder and is characterized by excessive dryness of the eyes, mouth, and other mucous membranes. White blood cells can infiltrate... read more 
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An abnormality in the tear-drainage system (canaliculi, tear sac, or nasolacrimal duct) can interfere with drainage and cause excessive tearing or lead to inflammation and infection.
