That pink bump in your eye is a leftover third eyelid
The small, pink nodule at the inner corner of your eye is called the plica semilunaris. It is a vestigial remnant of the nictitating membrane, or "third eyelid," which still functions in birds, reptiles, and sharks. While humans no longer need a translucent membrane to sweep across the eye for protection, the structure remains behind as an evolutionary relic.
Anatomy of the Inner Canthus
When looking closely at the human eye in a mirror, the inner corner nearest the nose reveals distinct anatomical features that differ from the rest of the ocular surface. Among these structures are the lacrimal caruncle—a small, fleshy, pinkish nodule—and resting immediately beside it, a delicate, crescent-shaped fold of conjunctival tissue known as the plica semilunaris. This fold sits within the medial canthus, lining the curved boundary where the upper and lower eyelids meet.
The plica semilunaris is composed of mucous membrane tissue continuous with the bulbar conjunctiva, the transparent layer covering the sclera or white portion of the eye. While it appears modest and unremarkable compared to the prominent upper and lower eyelids, this tiny fold represents a complex anatomical bridge between human ocular physiology and the evolutionary history of vertebrate vision.
The Functional Nictitating Membrane
In many other vertebrate species, the tissue corresponding to the plica semilunaris is neither small nor passive. It forms a fully developed, active structure called the nictitating membrane, commonly referred to as the third eyelid. Unlike human eyelids, which open and close vertically, the nictitating membrane is positioned to sweep horizontally across the surface of the cornea from the inner corner outward.
This membrane serves several vital functions across different classes of animals, including birds, reptiles, amphibians, and certain fish such as sharks. Because it is typically translucent or transparent, an animal can deploy the nictitating membrane to protect the delicate corneal surface while maintaining a degree of vision. In predatory birds, for instance, the membrane clears dust and shields the eyes during high-speed dives, while in aquatic creatures, it acts as a built-in protective barrier against debris and water pressure.