Skeletal system: structure

An adult skeleton has 206 bones, but you were not born with that number. A baby has more than 270 bones and pieces of cartilage, and many of them fuse together as the child grows. Your skeleton is not just a frame, either. Every bone is a living organ with blood vessels, nerves, and cells that keep repairing it.

The skeleton has two parts. The axial skeleton runs down the center of the body: the skull, the spine, the ribs, and the sternum (breastbone). The appendicular skeleton attaches to it: the arms, the legs, and the shoulder and hip bones that connect them. Bones also come in shapes that suit their jobs. Long bones, like the femur in your thigh, act as levers. Short bones, like those in your wrist, allow small, precise movements. Flat bones, like the skull and ribs, protect organs. Irregular bones, like the vertebrae, do several jobs at once. Each of your hands has 27 bones, which is why it can do so many different things.

Cut a long bone open and you can see layers. The periosteum is a tough outer membrane that carries blood vessels and nerves. Beneath it, compact bone is hard and dense. Inside is spongy bone, which looks like a honeycomb of tiny struts. It is much lighter than solid bone but still strong, like the framework of a bridge. Soft bone marrow fills the spaces inside. Bone is hard because of minerals such as calcium and phosphorus, and it is flexible because of collagen, a protein fiber. Together they make bone strong without making it brittle.

Bones meet at joints. Some joints, like the ones in the skull, are fixed. Others move. A ball-and-socket joint, as in the shoulder and hip, allows movement in nearly every direction. A hinge joint, as in the elbow and knee, moves like a door in one direction. A pivot joint at the top of the neck lets you turn your head to say "no." A gliding joint in the wrist lets small bones slide past each other. Three tissues keep joints working: ligaments connect bone to bone, tendons connect muscle to bone, and smooth cartilage cushions the ends of bones and reduces friction.

A common mistake is to think bones are dry and dead. The brittle bones in a museum have had their living cells removed. The bones in your body are alive, grow, and heal. Next lesson: what the skeleton does for you.