membrana nictitans

pli·ca sem·i·lu·na·'ris of con·junc·ti·'va

[TA] 1. the semilunar fold formed by the palpebral conjunctiva at the medial angle of the eye; 2. a fold of the conjunctival mucous membrane found in many animals; normally partially hidden in the medial canthus of the eye when at rest, it may be extended to cover part or all of the cornea in a winklike action to clean the cornea, as in birds. Synonym(s): membrana nictitans, nictitating membrane, palpebra III, palpebra tertia, third eyelid Synonym(s): plica semilunaris conjunctivae [TA], plica lunata, plica semilunaris of eye, semilunar conjunctival fold

membrana nictitans

(1) Semilunar fold of conjunctiva; plica semilunaris conjunctivae [NA6]. 
(2) Nictitating membrane.