Babies are born with about 330 bones, but many of them join together during the process of growing up, creating fewer, larger bones. Adults have 206 bones. Some people end up with a few extra bones, though, in the arches of their feet or as an extra set of ribs in their rib cage.
Many bones are shaped to protect and support soft body parts. The curved bones of your skull, for instance, enclose and protect your brain, your body’s command center. The ribs form a cage that protects your heart and lungs. Your wrists, hands, ankles, and feet contain more than half the bones of your body. Usually the more bones, joints, and muscles you have in a spot, the more flexible it is. That is why you can make such small, precise movements with your hands and feet, like tying a bow or balancing on your tiptoes.
Your skeleton makes up the hard, strong framework of your body. Your skeleton can move easily because it consists of many bones connected by flexible joints. Joints are kept together by elastic straps called ligaments and by a smooth, flexible tissue called cartilage, which covers and joins the ends of bones. Muscles are usually attached to bones by other tough elastic straps, or bands, called tendons. During movement, when muscles contract, it is the ropelike tendons that pull bones into their new positions.
The longest and strongest bone in the human body is the thighbone, or femur. In a man six feet (1.8 meters) tall, the femur would measure about 20 inches (51 centimeters) in length. Leg bones are very strong because they have to carry the weight of the body and move it from one place to another.
The smallest bone in your body is the stirrup, located in the middle ear. It is about the size of a grain of rice. It is named that because it looks just like a tiny stirrup, the part on a horse’s saddle on which you rest your foot. When sound waves enter your ear canal they vibrate your eardrum. The three tiny bones behind it also vibrate, increasing the strength of the sound as it passes through the middle ear. The stirrup, along with the hammer and the anvil bones also named for their shapes transfer their vibrations to the inner ear, where they become nerve impulses and travel to your brain.
Bones break when too much stress is applied by physical forces. Breaks usually occur when a person is injured playing sports, or when he or she is involved in an accident. Bone diseases can also cause breaks. With serious breaks called compound fractures you know something is wrong because the bone sticks out through the skin, and muscles and other tissue are damaged around it. But a lot of bone breaks are simple fractures, where the bone has broken cleanly instead of splintering, and it doesn’t push out through the skin. In such cases it is hard to know if you have broken a bone, or if the pain you’re feeling is caused by a pulled muscle or a sprained joint.
A fractured bone usually causes a lot of pain, and the injured area may look swollen or be shaped funny. Still, the only way to tell if you have a broken bone for sure is to have it X-rayed. X rays, short-wave radiation that can pass through flesh but not bone (leaving a picture on photographic film or plates), can detect a break in a bone.
If your bone is broken, the pieces will have to be held together in the right position in order to heal properly, growing back together as before. Sometimes a hard cast or splint is used to keep the injured body part from moving for a few weeks. Some broken bones are held together naturally, by the muscles that surround them. And in serious cases, metal screws, nails, and plates keep the ends of broken bones in place as they heal. Healing time depends on the bone broken and the age of the injured person. The bones of children heal very quickly.
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