Definition: 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.
ptosis of X ref THIRD EYELID: coord IM with EYELID DISEASES /vet (IM), not BLEPHAROPTOSIS
Other names Third Eyelid; Third Eyelids; Nictitating Membranes; Membranes, Nictitating; Membrane, Nictitating; Eyelids, Third; Eyelid, Third