When we enter into a dusty area, our two eyelids, the upper and lower, will blink to protect our cornea, iris sclera, and all the other sensitive parts. But what if we told you that a third lid exists and that you’ve probably seen it every time you’ve looked into the mirror?
Like reptiles, birds, and other amphibians, humans have a third eyelid sitting in the inner corner of their eye.
It’s true.
The remnant of our third eyelid is called “plica semilunaris,” but humans don’t use this layer.
In reptiles, the eyelid slides across the eye as an additional layer of protection. So, next time you look into the mirror, see if you can recognize your plica semilunaris without poking your eye.