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.