Plica semilunaris

The plica semilunaris is an accessory organ of the eye. It is the crescent shaped fold located in the medial angle lateral to the lacrimal caruncle. For most mammals, it is a vestigal organ (such as the third eyelid in birds). Once thought to be related to the nictitating membrane; it turned out to be structurally and functionally different. For humans, it even serves an important purpose. It makes eye boogers! The process is very similar to that of the mucus membrane in the nostrils. A fatty substance is secreted, which saps up foreign materials such as dirt and dust, which could scratch the cornea. The congealed mass is then deposited into the corner of the eye, waiting be brushed away in the morning.

Although the plica semilunaris is not exactly a vestigial organ, since it does serve a purpose, the purpose is just a fraction of what the nictitating membrane (or "third eyelid") does for other animals. It does help lubricate the eye and provides mucus to collect foreign material and create "dry pellets" or eye boogers that also collect at the medial canthus, but unlike its more developed cousin, the nictitating membrane, it has lost its ability to retract. In fact some of the few mammals that still have a nictitating membrane, like cats and dogs, have lost most of the muscle fibers to actively retract it, which birds still have. (See Ophthalmology for the Veterinary Practitioner)