Here’s a bit more about third eyelids in animals and even humans

Whether or not humans have nictitating membranes — the so-called third eyelid — has been a source of argument among those who study ocular anatomy.

Nictitating membranes are a prominent part of the eye in some animals. It crosses the eye horizontally instead of vertically both sweeping tears and protecting the cornea. Depending on the animal, including our pets, it is either transparent or translucent. Humans have not been known to have a third eyelid per se.

In 2017 however, a paper was published documenting a case of a third eyelid and its surgical removal, in a 9-year-old girl. Since birth, the membrane originated in the corner of the eye nearest the nose. It continued across her eye and covered about a third of her pupil obstructing her vision.

The excess tissue’s surgical removal was easy and healing was uneventful. The tissue did not grow back.

So, what was a third eyelid doing in a human? Most refer to such bits of tissue as vestigial. The word means a structure that was once much larger and more noticeable which has since shrunk and become mostly disused across a species.

We and other members of the animal kingdom have several vestigial structures. Take, for example, our third molars also known as wisdom teeth. We don’t need them.

In some of our wrists, if you rotate your forearm outward with the palm up and then flex your hand up and toward your chest, you may see a prominent tendon standing taut in the middle of your wrist where it turns into the hand. That tendon is part of the palmaris longus muscle, a very weak wrist flexor which we don’t need any longer. In humans, estimates are that about 10% are born without one or both muscles.

Sometimes we thought for a long time that some things are vestigial and unnecessary. Take our appendix. This little fingerlike, blind-ending projection off of our bowel can get inflamed and need to be removed. In days long gone, if a surgeon had to enter someone’s abdomen for other reasons, they would take out the appendix, “just in case.”

Since the study of the trillions of microorganisms that normally inhabit our bowel has increased significantly, some medical researchers believe this little worm-of-a-structure may be a necessary reservoir for vital bowel organisms. This coupled with litigation for taking things out without consent has stopped all “since we were there,” surgical add-ons.

Perhaps the most important thing here is to understand why some anatomists are saying, to some extent, we humans all have some vestigial third eyelids present on the inside corners of our eyes. And there are a sizeable number who fundamentally disagree.

This happens because often in the sciences and medicine one can observe differences of opinion separating into stalwart camps. There are the so-called “separationists” who are constantly looking for the slightest issue, tissue, or toenail (my words here) to segregate animals into separate, distinct species, sub-species, and breeds.

The other camp is the “bunchers,” who tend to feel that such indexing is often weakly supported. They will let the DNA speak for their assertions, but even that can get complicated and arguable. This is how a civilized society hones the cutting edge of science and our critical understanding of one another.

If you want to see if your eye resembles that in your pet dog or cat, simply lean into the mirror and look at the inside corner of your eye. There is a small, rounded bit of tissue there that while part of the conjunctiva is indeed microscopically the same as a third eyelid in an animal.

Here’s just a quick note to express my gratitude to the publishers and editors of the Lewiston Tribune and the Moscow-Pullman Daily News and our readers for their patience during my recent illness.

I suffered an age-related spinal cord injury between the first and third vertebrae in my neck. I was lucky to have gotten Life Flight transport and definitive care in Spokane. In mid-December, a neurosurgeon surgically stabilized my neck with rods and screws. Thankfully, I’ve gotten most of my function and this column back.

Powell, of Pullman, retired as public information officer for Washington State University’s College of Veterinary Medicine in Pullman. This column reflects his thoughts and no longer represents WSU. He may be contacted at charliepowell74@gmail.com.