What Are Bones Made Of?

If you've ever seen a real skeleton or fossil in a museum, you might think that all bones are dead. Although bones in museums are dry, hard, or crumbly, the bones in your body are different. The bones that make up your skeleton are all very much alive, growing and changing all the time like other parts of your body.

Almost every bone in your body is made of the same materials:

•     The outer surface of bone is called the periosteum (say: pare-ee-OS-tee-um). It's a thin, dense membrane that contains nerves and blood vessels that nourish the bone.
•     The next layer is made up of compact bone. This part is smooth and very hard. It's the part you see when you look at a skeleton.
•     Within the compact bone are many layers of cancellous (say: KAN-sell-us) bone, which looks a bit like a sponge. Cancellous bone is not quite as hard as compact bone, but it is still very strong.
•     In many bones, the cancellous bone protects the innermost part of the bone, the bone marrow (say: MAIR-oh). Bone marrow is sort of like a thick jelly, and its job is to make blood cells.