Like buildings, the bodies of man and other backboned ani­mals are supported by a firm framework, This framework is made of bone and is called a skeleton. The skeleton supports the skin, muscles, and other body parts.
   Bone has two main parts—a close, hard outer layer, with an open web-like, spongy part inside. You can see both parts in sawed-up beef bones used for making soup. Bone is living tissue, just like other parts of the body.
   Young people's bones grow larger along with the rest of the body. Bones stop growing when full growth is reached, usually when a person is 18 to 20 years old. But bone cells continue to be replaced as they wear out. In this way, bone is like other body parts, where worn-out cells are continuously being replaced.
   Bone contains calcium and phosphorus. These minerals give bone its hardness. Bone also contains a jelly-like substance that gives it some springiness. Bone can bend some-what, but it cracks or breaks if the bending force is very strong. Get two chicken leg bones. Soak one in a jar of vinegar for 48 hours. When the soaking time is about up, put the other bone in a pan and heat it in a very hot oven (about 500° F) for an hour. When the bone cools, break it. All the soft, jelly-like material has been "cooked" out of the bone. Only brittle minerals re-main. Take the first bone out of the vinegar and try to break it. What happens? Vinegar dissolved min­erals in the bone, leaving it springy.
   Broken bones normally heal. New bone cells form around the break. The pieces join perfectly when the break is properly set by a doctor.
   Look at a bone in a slice of ham or roast beef. The bone is like a hard tube. The blood vessels and nerves, which feed and give sensation to the living bone, flt inside the tube, to keep it healthy and let it grow nor­mally. A soft material called marrow is also in the center of the tube. Red blood cells are made in the marrow of some bones.