Why Do We Need a Skeleton?為什麼我們需要骨骼？

🔊Imagine for a moment that every bone in your body disappeared. You could not stand, sit up, or even hold up your head. You would slump to the floor like a pile of laundry. Some animals, such as jellyfish and octopuses, get along fine without bones, but they live in water, which helps hold them up. On land, a large animal needs a strong frame inside it. Your frame is your skeleton: about 206 bones, from the tiny ones deep inside your ears to the long thighbone in each leg.

🔊The first job of the skeleton is support. Your spine, a stack of 24 small bones called vertebrae resting on a larger bone called the sacrum, holds up your head and body like the pole of a tent. Your pelvis passes that weight to your legs, and your legs carry it down to your feet. The second job is protection. Your skull is a hard helmet around your brain. Your ribs form a cage around your heart and lungs, and your vertebrae make a bony tunnel that shields your spinal cord, the bundle of nerves that carries messages between your brain and the rest of your body.

🔊The third job is movement. Bones cannot move by themselves, but muscles are attached to them, and when a muscle gets shorter, it pulls on a bone. The places where two bones meet are called joints. Some joints, like your elbow and knee, bend in one direction, like a door on its hinges. Others, like your shoulder and hip, are ball-and-socket joints that let you swing an arm or a leg in a circle. Your hands are especially clever. Each one has 27 bones, which is why you can write, button a shirt, and pick up a peanut with chopsticks.

🔊The fourth and fifth jobs are hidden inside. Many bones are filled with a soft tissue called marrow. Red marrow, which in adults is found mostly in flat bones such as the ribs, hips, and vertebrae, makes new blood cells, hundreds of billions of them every day. They include red blood cells that carry oxygen, white blood cells that fight germs, and platelets that help a cut stop bleeding. Bones also store minerals, especially calcium. When your heart and nerves need more calcium than your food provides, your bones release some into your blood.

🔊It is easy to think of bones as dead, dry sticks, like the skeleton that hangs in a science classroom. But the bones inside you are alive. They have blood vessels and nerves, which is one reason a broken bone hurts so much, and they are always rebuilding themselves. Some cells break down old bone while others build new bone, so over about ten years, most of an adult's skeleton is replaced. A newborn baby has around 300 bones and bony parts. As a child grows, many of them join together, which is why an adult has only about 206.

🔊Because bone is alive, it responds to how you treat it. When you run, jump, and play, the extra force tells your bones to grow thicker and stronger. Astronauts who spend months in space, where their bones carry almost no weight, lose bone and must exercise every day to slow the loss. Your bones reach their greatest strength by your late twenties, and much of it is built while you are still in school. So move every day, and eat foods with calcium, such as milk, tofu, small dried fish, and dark green vegetables. Then try this: count the bones in one of your hands.