part of part of in taxon Haplorrhini Serpentes eyelid conjunctiva nictitating membrane nictitans SCTID:113326005 haw UBERON:0010207 NCIT:C77657 Wikipedia:Nictitating_membrane third eyelid A fold of the mucous membrane of the conjunctiva in many animals. At rest, it is hidden in the medial canthus. It can extend to cover part or all of the cornea to help clean the cornea. MESH:D009541 Wikipedia:File:Chickenblinking.jpg