Plica semilunaris of the conjunctiva

The plica semilunaris is a small fold of bulbar conjunctiva on the inner corner of the eye. It is loose, so that eye movements are not restricted.

It is the vestigial remnant of the nictitating membrane (the "third eyelid") which is present in other animals.

The plica semilunaris also secretes rheum, commonly known as sleep: the hardened crust that forms on the eye during sleep.