[upbeat electronic music]

- [Stefan] You might think that you're pretty familiar with your body. I mean, you live in it every day. But it turns out that our bodies still have some surprises for us. We have body parts you've probably never heard of and likely haven't even noticed, including some or all of these three.

First, you might have a little-known body part embedded in a muscle specifically near your knee. Your knee joint is made of the femur, the tibia, and the patella, also known as the kneecap, but some people have an extra bone hidden in a muscle that runs along the back of the leg. It's called the fabella, which means "little bean" in Latin, and it's between 5 and 20 millimeters long.

Because they're so small, these bones are really hard to detect in X-rays and CT scans, so we're not sure how many people have them. Studies report that 3 to 87% of people may have this bonus bone, which is quite the range. And among those people, 20 to 30% have a fabella in only one leg. It's not clear why, so we need more research on these little beans and why they're distributed the way they are.

That said, we are pretty sure what the fabella does. The leading idea is that its job is to stabilize a bony protrusion at the end of the femur and to distribute muscle forces around the knee joint. This is similar to the role of the kneecap, except it's on the back of the knee. But despite potential benefits, this little bone bean can cause big problems, like severe knee pain. The fabella can put pressure on a nerve, it can fracture, and it can compress nearby bones, ligaments, or tendons, which is all the more reason to better understand it.

Now, the fabella isn't the only obscure body part that affects people in different and quirky ways. Around each of our ears, there's a set of four muscles known as the auricular muscles. They exist to move the pinna, the outer ear-shaped part of the ear, or at least they used to. Ear movement is super important for animals, because it allows them to orient their ears in the direction of sounds or to express emotions. But humans don't do this, so for us, these muscles are vestigial, which means they've lost their functionality over the course of evolution.

About 20% of people can use these muscles to wiggle their ears, though, and typically this talent doesn't have a beyond a neat party trick, but in recent years, scientists have started exploring whether people with disabilities or paralysis could be trained to use these muscles to operate assistive devices. Computers can turn the electrical signals that happen when someone moves their auricular muscles into commands for prosthetic devices or electric wheelchair. For example, moving a muscle on the right ear could tell a wheelchair to roll forward, and moving one on the left could tell it to turn.

A similar technique has been tried using nose and tongue movements, but that tends to interfere with other things, so some researchers think auricular muscles could offer the perfect solution. See, the nerves that control the auricular muscles don't actually enter the spinal cord, which means they aren't affects by a paralysis-causing spinal cord injury. Plus, since they're not used for anything else, there wouldn't be any confusion over whether someone is trying to signal their device or just smell some baking bread. And because auricular muscles are kind of isolated, a computer would be less likely to accidentally pick up on signals from other muscles, too.

And finally, ears aren't the only sensory organs with little-known parts. Eyes have them, too. About a pencil-width in from the inner corner of your eye are tiny holes called the lacrimal puncta. Each eye has a punctum on the top and bottom lids. They're part of the lacrimal system, which creates tears for lubricating and cleaning the eyes and also for crying.

If you've noticed these holes, you might've guessed they produce tears, which is fair, but really, tears are produced by the lacrimal gland, which is located above and behind the eyeball. The tears flood over the eyes and collect in pools in the inner eye, but from there, they do drain out from the puncta into small tubes. Eventually, the tears flow to the back of the nose, where they mix with mucus and can cause your nose to run if you're crying a lot.

Now, the lacrimal puncta can get blocked. A blockage can be present from birth or be caused by infection, medication, or just the eyelid turning outward as you age and your muscles and ligaments relax. If that happens, or if the puncta are naturally too small, tears can't drain into the nasal cavity, and you can get watery eyes. So in severe cases, doctors can actually stretch the puncta or reopen them with a tiny incision. On the other hand, a treatment for dry eyes is inserting plugs that block the puncta.

And all of this is a reminder that even the most unsung parts of our anatomy can be really important. Like, the three body parts on this list might be obscure or small, but all of them can have huge impacts on our quality of life.

Now, before you go, we wanted to let you know that this is the last week for our February pin of the month, so if you've been thinking it over or want to check it out, the time is now. This month's pin is the Leviathan of Parsonstown, an 1800s telescope that completely lives up to its name. You can learn more about it over at SciShow Space, and you can find the pin at DFTBA.com/SciShow until March 1st.

[upbeat electronic music]