Eyelid

A movable fold of skin that covers and protects the front of the eye. Blinking spreads tears across the eye’s surface and helps keep it moist.

Blink reflex: An involuntary response that closes the eyelids when the eye is threatened or stimulated. This protective reflex rapidly closes the eyelid against debris, bright light, or contact.
Tarsal plate: Dense connective tissue that provides structural support to each eyelid. It stiffens the eyelid while allowing it to move over the eye.
Ptosis: Abnormal drooping of the upper eyelid. A weakened or impaired eyelid elevator can cause the upper lid to droop.
Nictitating membrane: A transparent or translucent third eyelid found in many vertebrates. Unlike human eyelids, this membrane can cover the eye while preserving some vision.
Orbicularis oculi: A facial muscle that encircles the eye and closes the eyelids. Its contraction produces the eyelid closure used in blinking and forceful eye protection.
Eyelashes: Short hairs along the eyelid margins that help detect and intercept particles. They form the eyelid’s first physical barrier against debris near the eye.
Blepharitis: Inflammation of the eyelid margins, often involving the oil glands or nearby skin. Inflammation at the lid edge can disrupt glands and irritate the eye surface.
Corneal reflex: An involuntary blink elicited by stimulation of the cornea. It is a specific sensory trigger for eyelid closure, distinct from the eyelid structures themselves.
Levator palpebrae superioris: A muscle that raises the upper eyelid. Its action opens the eye by lifting the upper eyelid.
Conjunctiva: A thin membrane lining the inside of the eyelids and covering the white of the eye. The inner eyelid surface is lined by this membrane, which continues onto the eye.

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