The bones inside your body make a hard frame called the skeleton. Bones hold up your body and give it its shape, and a grown-up's skeleton has 206 bones. You might think bones are dry sticks that never change. In fact, bones are living tissue, and old bone is constantly broken down and replaced by new bone, a process called remodeling. Let's see what else bones can do.
Long bones grow longer at growth plates near their ends, not from the middle. A child's growth plates harden into solid bone once the child is fully grown, and then the bones stop getting longer. Bones also store calcium, a mineral, and give some back to the blood when the body needs it. Bones get stronger when you use them by being active.
Look inside a bone. Bone is a framework of the protein collagen made hard by the mineral calcium phosphate. The outside is made of hard, dense bone, and soft bone marrow inside many bones makes most of your blood cells. Bones come in four main shapes: long, short, flat and irregular.
A joint is a place where two or more bones meet. Hinge joints, like the knee and elbow, only bend and straighten. Ball-and-socket joints, like the hip and shoulder, can move forward, backward, sideways and in circles. Cartilage covers the ends of bones in a joint and helps them move without rubbing. Most muscles are attached to bones, and the bones act as levers that let body parts move. Tendons connect muscles to bones.
How can anyone see a bone that is inside a body? Doctors use X-rays to see whether a bone is broken. Wilhelm Röntgen's X-ray picture of his wife's hand showed the bones of her hand and her ring. When a bone does break, a cast or splint holds it still while it heals. Children's broken bones can heal well and heal quickly.
Some bones are guards. The bones of your skull protect your brain, and your ribs form a cage that protects your heart and lungs. A baby's skull bones are not firmly joined at birth, which leaves soft spots on the head. Not every skeleton is the same, either. The part of your ear you can see is made of cartilage, a tough bendy tissue, not bone.
A shark has a skeleton made of cartilage instead of bone. A giraffe has seven bones in its neck, the same number as a person. A bird's bones are hollow and light, which helps it fly. So skeletons come in more than one kind.
Words to know
- remodeling: old bone replaced
- collagen: bone protein
- marrow: soft inner part
- cartilage: bendy tissue
