Why do I have that pink corner of the eye? Plica semilunaris: The Third Eyelid

If you look at your eye, you’ll notice that in the corner nearest to your nose you have a little pink triangular structure. In medical speech this little triangle is made up of two parts: the little bump is the lacrimal caruncle and the nearby crescent-shaped area bordering the eyeball is the plica semilunaris. Why do we have this strange pink area of the eye?

The tiny crescent-shaped area within the pink corner of the eye known as the plica semilunaris has been the subject of much discussion and debate, especially amongst evolutionists and creationists. Why? Because this area of the eye is thought to be a remnant from our evolutionary history, when we had a spare eyelid that shot out from this area!

Although we no longer have a spare third eyelid that can slide horizontally across our eyes, reptiles, amphibians, fish, birds and some mammals (polar bears, seals and aardvarks to name a few) do still have this interesting translucent structure, whose technical term is the “nictitating membrane”. In these animals it is used to clean and protect the eyes much like our eyelids do. It is also used to protect the eyes from the sun or glare, acting like sunglasses, or it can act like goggles when the animals are under water.

Evolutionists say that the presence of this pink crescent in the corner of our eyes is proof that we evolved from creatures that once had a functional nictitating membrane. Their argument is that we have retained this now-pretty-much-useless vestigial organ because it hasn’t yet been selected against by evolution.

Some creationists say that the plica semilunaris is not proof of evolution because it functions to make and collect “sleep” that we get in our eyes in the morning. Sleep is secreted from a combination of sources including the lacrimal gland, conjunctival goblet cells, the meibomian gland, in some cases also from nasal mucous overflow, and perhaps a little from the nearby lacriminal caruncle. The plica semilunaris may contribute to the secretions because it is rich in goblet cells which make mucus.

So why do we have the pink corner of the eye? Well, both the lacrimal caruncle and the plica semilunaris contain cells which allow it to secrete things which may contribute to our tears and their functions in protecting our eyes, keeping them moist and healthy, but could the plica semilunaris be a remnant of a third eyelid? If it is, what a shame that we didn’t retain it! I for one think it would have been great to have built-in sunglasses and underwater goggles!

References
Cairo University’s Faculty of Medicine on the anatomy of the plica semilunaris and laminal caruncle

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