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!" )