An eyelid was a flap of tissue that protected an individual's eyes from irritants and light.
Vulcans were known to have an inner eyelid which protected their eyes from the harsh sunlight on planet Vulcan. While traversing Vulcan's Forge, T'Pol assured Jonathan Archer that hers would protect her from the sun's glare. (ENT: "The Forge")
In 2267, on a mission to the Deneva colony, Spock's inner eyelids enabled him to completely recover his sight after being blinded by a bright light dose intended to free him from a neural parasite. (TOS: "Operation -- Annihilate!")
In 2368, Geordi La Forge remarked to Hannah Bates that he felt as though his eyelids had lead weights attached. (TNG: "The Masterpiece Society")
In 2372, when Jake Sisko tried to awaken his grandfather from a nap, the older man facetiously asserted that he was not sleeping, just checking his eyelids for holes.(DS9: "Homefront")
Displacement of the eyelids was a possible symptom of Tuvan Syndrome. In 2375, Doctor Julian Bashir told Luther Sloan that Koval had this symptom (DS9: "Inter Arma Enim Silent Leges")
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