LAB Medicine - file 01
Your lost third eyelid - Plica Semilunaris
Look into your eyes in the mirror. You can soon see a small, pinkish, crescent-shaped fleshy fold in the inner corner, just like wrapping your eyeball from the backside. This looks like a part of the muscles that moves your eyeball. But actually NOT a muscle. This is called the Plica Semilunaris (“crescent fold” literally in English) in anatomy.
What is the Plica Semilunaris?
The Plica Semilunaris is a small, vertical fold of the Conjunctiva (the thin, clear membrane covering the white of your eyes, often bloodshot). They are NOT muscles engaging in the movement of eyeballs, but a Vestigial Organ, which means a leftover from our evolutionary ancestors that no longer performs the original functions, such as the Coccyx (so called tailbone).
The “Third Eyelids”
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| Nictitating membrane (source: PixaHive) |
In other animals, such as cats, birds, fish, and reptiles, this structure is a fully functioning Nictitating Membrane, so called “third eyelids.” These animals can close and open this membrane horizontally (NOT vertically, unlike the eyelids) across the eye to protect from debris or dust, to keep it moist, and to shield it while hunting from harm. And all the while, it can maintain some degree of vision as the third eyelids are nearly transparent or opaque.
On the other hand, in humans as well as most of the primates, we’ve lost the muscles and the structural needs for the third eyelids. So as a result of the retrogression, we still have only a pair of tiny folds of the tissue…
So is the crescent fold useless today?
Even though vestigial, it is NOT altogether useless. The Plica Semilunaris directs tears toward the Puncta (the drainage holes) in your eyelids. Also it provides a little extra slack in the conjunctiva, like an accordion, to allow your eyeballs to rotate inwards toward your nose without the tissue feeling tight or restricted.
Today, the Plica Semilunaris has 3 main jobs;
1. Mechanical slack for eye rotation
Imagine the bellows of an accordion, or of a flexible pipe for a washing machine. When you rotate your eyes inwards toward your nose, the conjunctiva (the transparent coating membrane of the eyeball) needs to stretch. So the Plica Semilunaris provides flexibility as an extra tissue.
If we had lost the Plica Semilunaris on the way of evolution, your conjunctiva would pull tight, restricting your range of eye motion and visible angles. Also, you would feel pain or discomfort when you look sideways.
2. Debris collection like eye slime
The Plica Semilunaris contains Goblet Cells that secrete mucus. This mucus helps catch dust, dirt, and dead cells on the eye’s surface. And then it funnels this debris toward the inner corner of the eye, mixing with the mucus to form “sleep” or “eye crusties.” This protects your Cornea (the transparent front cover over the iris and pupil) from being scratched by floating particles.
3. Tear drainage support
The Plica Semilunaris also helps maintain the “Lacrimal Lake” (a poetic word, isn't it? But actually a technical term in medicine) that is a small pool area in the inner corners of the eye. Holding the mixed fluid in place it makes sure that tears can be efficiently sucked up into the puncta (the drainage holes) leading to your nose.
In biology, “vestigial” often gets confused with “useless,” but as the Plica Semilunaris testifies, evolution is very good at repurposing old parts for new, lesser functions. So this is true evidence that we have really evolved to adapt over millions of years of mind-boggling biological history.
Anyway,... can you imagine yourself having a third eyelid besides a tail?!
Further reading (sponsored by Amazon):
“Clinical Anatomy of the Eye”
(sponsored by Amazon)
Table of Contents
3. The orbital cavity
4. The paranasal sinuses
5. The ocular appendages
6. The eyeball
7. The anatomy of the eyeball as seen with the ophthalmoscope, slit lamp, and gonioscope
8. Movements of the eyeball and the extrocular muscles
9. The orbital blood vessels
10. Cranial nerves part I: Those nerves directly associated with the eye and orbit
11. Cranial nerves part II: Those nerves not directly associated with the eye and orbit
12. The autonomic nervous system
13. The visual pathway
"Ocular Anatomy and Physiology"
(sponsored by Amazon)
Updated to include NEW material for beginners in ophthalmology and optometry, “Ocular Anatomy and Physiology (second edition)” is an essential text that covers a range of fundamental information for students and clinicians!
Table of Contents
"Kanski's Clinical Ophthalmology: A Systematic Approach"
(sponsored by Amazon)
Through 8 outstanding editions, "Kanski’s Clinical Ophthalmology" has been the classic specialty textbook, providing the perfect ophthalmology foundation for trainees and a valuable reference source for experienced practitioners!
