The
plica semilunaris is a small fold of
bulbar conjunctiva on the medial canthus of the
eye. It functions during movement of the eye, to help maintain tear drainage via the
lacrimal lake, and to permit greater rotation of the
globe, for without the plica the conjunctiva would attach directly to the eyeball, restricting movement. It is the
vestigial remnant of the
nictitating membrane (the "third eyelid") which is drawn across the eye for protection, and is present in other animals such as
birds,
reptiles, and
fish, but is rare in
mammals, mainly found in
monotremes and
marsupials. Its associated muscles are also vestigial. It is loose, thus eye movements are not restricted by it. Only one species of
primate, the
Calabar Angwantibo, is known to have a functioning nictitating membrane.