Warning: some of you might find the following illustration very frightening. Viewer discretion is advised!
This is a skeleton. It’s a popular Halloween decoration . . . because it’s so scary! Some people probably even find it scarier than other scary Halloween things—ghosts and wolf-men and Draculas.
But I’ve always thought it’s a little strange to be scared of the human skeleton, Junior Geniuses. It’s like being scared of peanut butter sandwiches when you’ve just eaten a peanut butter sandwich. I don’t want to alarm you, but . . . THERE IS A BIG SCARY SKELETON INSIDE YOU RIGHT NOW!
You should be glad to hear this. Without your bones you would be a shapeless lump of flesh, unable to talk or play mini golf or even hold a pencil. Let’s thank our skeletons by learning a little bit about what they are and how they move.
JUNIOR GENIUS JOVIALITY!
Q: Why don’t skeletons fight?
A: They don’t have any guts.
If you want to see your skeleton, the best option used to be vultures. Hungry vultures can strip a corpse down to bare bone in just five hours! Nice job, vultures. That is some quick work there.
But now there’s an even faster—and less gruesome—way to see what your skeleton looks like. In 1895 a German scientist named Wilhelm Roentgen accidentally discovered a new kind of radiation that could pass through cardboard. He didn’t know what these new unknown rays were, so he called them X-rays. As one of his first experiments, he put his wife’s hand on a photographic plate and exposed it to X-rays. The resulting picture was a bony skeleton hand wearing a ring. “I have seen my death!” said his shocked wife. But instead, X-rays brought life: countless people have been saved by radiology, Roentgen’s new science for imaging the inside of the human body.
Medical students who spend a lot of time studying the human skeleton often cut Doritos out of their diet. Why? Because of the smell! When students saw through skulls and limbs, they find that human bone dust smells weirdly like corn chips.
Only 15 percent of your body weight comes from bones. This is partly because your bones have spaces inside them. (The insides are filled with a gunk called marrow, which is where blood cells are made.) But despite their surprisingly light weight, bones are incredibly strong—stronger than steel, for their weight. A bone the size of a matchbox could hold up nine tons of weight, which is four times what the same size of concrete could support.
What makes bones strong? Minerals! Two-thirds of human bone are made up of crystals of calcium, the same element you find in rocks such as limestone and chalk. The other third is mostly a protein called collagen. If you soak a bone in acid, the minerals will dissolve and leave only the collagen. The bone will be so soft that you can literally tie it into a knot.
Calcium in bones is why grown-ups are always after you to drink your milk. At your age your skeleton is still growing every day, and more than 85 percent of girls and 64 percent of boys ages 12 to 19 don’t get enough calcium in their diet. You probably need about thirteen hundred milligrams of calcium a day, which is a lot. That’s:
So remember your dairy, my friends. (Or, if dairy is not a possibility for you, lots of leafy green vegetables. Mmm, broccoli.)
Broken bones are on the rise in kids nationwide, and calcium deficiency may be part of the problem. But no matter how many trees you fall out of, or how much milk you leave undrunk at lunch, you will probably never beat the record of Evel Knievel, who broke more than 433 bones during his lifetime! Knievel’s many health problems had nothing to do with bone density or yogurt. He was a professional daredevil who spent his life crashing his motorcycle into things. Good thing he always wore a helmet!
You have 206 bones in your entire skeleton, from the tip of your skull down to the soles of your metatarsals (your feet). You need them all, but here are a few especially interesting ones:
The cranial and facial bones. Your skull is actually made up of two sections, the cranium and the face . . . not one bone but twenty-two different ones, most of which meet at immovable joints called sutures. In the nineteenth century many scholars believed that you could tell everything about a person by feeling the bumps on his or her skull. This goofy “science” was called phrenology, but today, of course, it’s been completely debunked.
The clavicle. Also known as the “collarbone,” on each side of your body this connects your shoulder blade to the breastbone. The clavicle is one of the most commonly broken bones in the human body.
The hyoid. This horseshoe-shaped guy in the front of your neck helps you swallow. Weirdly, it’s the only bone in your skeleton that’s not attached to any other bones!
The ribs. Contrary to popular belief, men and women have the same number of ribs: twelve pairs each.
The ossicles. These tiny bones in your ear pass sound along from your eardrum by vibrating. They’re called the hammer, the anvil, and the stirrup because of their shapes. The stirrup, just three millimeters across, is the smallest bone in the body.
The femur. On the other hand, the femur, inside your thigh, is your longest bone.
The tarsals, metatarsals, and phalanges. One-quarter of all the bones in your body are found in your feet!
HOMEWORK
Your skeleton is weirdly symmetrical. Stick your arms out at your sides and measure your full arm span, from tip of middle finger to tip of middle finger. Compare that distance to your height. In most people it’s almost exactly identical!
If you measure your height in the morning and then again at night before bed, you’ll be surprised to find that you’re shrinking! That’s right, people are generally about an inch taller in the morning. Spending all day walking around with earth’s gravity pushing down on you tends to squish your skeleton, by curving your spine and compressing your joints slightly.
Joints are the places where bones meet, and they allow our skeleton to move. There are lots of different kinds.
If the following table does not display properly on your device, please click here to see it as an image.
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For Example: |
Not Unlike: |
Ball-and-socket joints |
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Hinge joints |
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Pivot joints |
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The space between bones at a joint is filled with a liquid called synovial fluid, which helps the bones slide around nicely. If you want to hear your synovial fluid at work, try cracking your knuckles. The sound you hear is caused by bubbles forming inside the joint when it gets extended—and then quickly popping.
Mom or Dad might be annoyed by the noise, but you can tell them that, according to several studies, cracking your knuckles isn’t bad for you. The best experiment was reported in 2009 by a man named Donald Unger, who spent more than fifty years cracking the joints on his left hand but never on his right hand. He never developed arthritis in either hand.
POP QUIZ
At your joints there’s a layer of stiff cushioning tissue that protects your bones—the same stuff that your nose and ears are made out of. Do you know what it’s called?
If all you had under your skin was a skeleton, then all you could do was stand there, like the skeletons in a medical college or a Halloween store. You would probably get bored, unless there was a TV nearby. Luckily, we all have something that lets us move our bones. We have muscles!
There are about 650 skeletal muscles attached to your bones, making up 40 percent of your body weight. That may seem like a lot, but don’t get too cocky, Junior Geniuses. The tiniest caterpillar has many more.
Muscles are attached to bones by tight little ropy things called tendons. Want to feel a tendon? The springy stuff connecting your calf to the back of your heel is the Achilles tendon, named for the warrior in Greek mythology who had his only weak spot there.
The Achilles tendon is a weak spot in real people too—it’s often injured by athletes, from middle-aged weekend warriors to pros like Kobe Bryant.
You can also see your tendons show by tightening the muscles in your arm or neck. Those gross strands that appear on the inside of your elbow and in your throat when you flex? Tendons.
Ligaments are similar to tendons, but they connect bones to bones, usually to stabilize important joints. In 2013 doctors in Belgium actually discovered a new knee ligament. This little guy, which the doctors called the anterolateral ligament, had been sitting in our knees all along, but it’s so thin and easy to miss that it had never appeared in medical textbooks!
Muscles go to work when your brain sends them electrical signals, telling them to contract. Muscles are made of thin filaments that can slide past each other, sort of like when you pull in a rope hand over hand. This process shortens the muscle, and the muscle contracts.
Let’s experiment with the muscles in our upper arm. The muscle on the inside of your arm, like all your muscles, has a fancy Latin name. It’s your biceps brachii. When you contract your biceps, your arm bends. Try it once! If you are now tired, please consider some time at the gym. It’s never too early to make good fitness choices.
But here’s the problem: the only thing your biceps can do is pull. If all you had in your upper arm were the biceps muscle, your arm would now be bent forever, like a mannequin’s. You would have only one bend per arm per lifetime. Use it well! But luckily, muscles come in pairs. On the outside of your upper arm is another muscle called the triceps. When that muscle contracts, your arm straightens.
Every muscle in your body is important. Just taking a single step requires moving two hundred muscles in perfect coordination. No wonder it takes babies a year or so to get the hang of it.
EXTRA CREDIT
Believe it or not, there are a few parts of the body that have no muscles at all. In fact, there are ten—your fingers! The muscles that bend your fingers and thumbs are all found in your palms and forearms. Only the tendons of these muscles go all the way into the fingers.
What’s the strongest muscle in your body? Some say it’s the tongue, but there’s no science to back that up. (Also, the tongue is made up of eight muscles rather than just one. But it is unique in the human body in that it’s only attached on one end!) Here are some better candidates for the bodybuilding title.
Your jaw. Because it’s so short, the masseter muscle in your jaw can bite down with incredible strength—975 pounds of force in some cases.
Your butt. All muscle fibers have about the same strength, so your larger muscles are, in a way, the strongest ones in total. And the largest muscle in the body is the gluteus maximus in each of your buttocks. You need it to run, jump, and climb. Butts aren’t just for sitting, my friends!
Your calf. No muscle pulls with greater force than the soleus, in your lower leg. Without it you couldn’t walk or even stand up straight. The average person walks far enough in their life to circle the earth three times, so the soleus has to be tough.
Your eye. As we’ve seen, no muscles are stronger for their size than the ones that move your eyes. And no wonder—an hour of reading requires nearly ten thousand muscle movements.
Your uterus. Granted, some of you may not have a uterus. You are missing out! This amazing organ of a woman’s body, where babies develop, is lined with a layer of muscle called the myometrium. When it’s time for the baby to be born, this muscle can squeeze with incredibly strong contractions.
WIMPY, WIMPY, WIMPY
As a contender for weakest muscle, how about the psoas minor? Located between your spine and your pelvis, the psoas minor is hidden by its larger cousin the psoas major. In fact, the psoas minor is so unimportant that half of all people don’t even have it!
There are actually two kinds of fibers in your skeletal muscles.
When I was your age, I would look at anatomical drawings and think, “Well, I see bones and muscles and organs and fat, but where’s the meat? Don’t people have meat?” Well, if you eat meat, the animal tissue you’re eating is mostly muscle. Muscle is meat.
And you know how at Thanksgiving the turkey has white meat and dark meat? The white meat in the breast is slow-twitch tissue, and not built for endurance. (Turkeys don’t fly much, so these muscles rarely get used.) The dark meat, in the legs and thighs, is colored by reddish myoglobin, which delivers oxygen to those fast-twitch muscles.
If meat is muscle, what would we taste like to a hungry bear or lion? Junior Geniuses, unlike many other children’s books authors, I do not endorse cannibalism. But the simple fact is that sometimes people eat other people. This might be for ritual reasons (certain South Pacific tribes) or survival or criminal ones (um, use your imagination).
In 1931 an American journalist named William Buehler Seabrook spent eight months with the Gueré cannibals of West Africa. In his book Jungle Ways he reported that human meat smelled like beef when cooked, but was lighter in color. It tasted “like good, fully developed veal.”
Our mix of muscle fibers explains why people can run both sprints (fast-twitch!) and marathons (slow-twitch). Lots of species, from cheetahs to deer to ostriches, can run faster than people, but we are much harder to beat over long distances. Because our ancestors survived by tracking speedy prey like antelopes, humans are better at endurance running than almost any other animal. Over marathon distances, trained runners have even beaten horses. Thanks, slow-twitch muscle!
Running a marathon and kicking a soccer ball are definitely important things we couldn’t do without muscles. But the skeletal muscles we use to move are just one kind of muscle tissue.
What about all the involuntary moving your body needs to do? You don’t have to think about it every time you need to push food from your esophagus into your stomach, or dilate an artery to change your blood pressure. These parts of your body are lined with a kind of muscle called smooth muscle, because its fibers don’t come in big stripy bundles like your skeletal muscles do.
In general it’s good that you don’t need to concentrate on contracting blood vessels or intestinal walls. But involuntary muscles can have their downside. You have a muscle called the diaphragm that helps you breathe air into your lungs. Which is great—until something short-circuits and the diaphragm starts contracting suddenly when it’s not supposed to. That’s when you get . . . the hiccups.
You actually have a third kind of muscle as well—involuntary like smooth muscle but as powerful as skeletal muscle. It’s called cardiac muscle, and it’s what your heart is made of. We’ll be taking a closer look at that a little later on. Now, I hope all that talk about cannibalism made you hungry, because it’s time for lunch.
