All bones are almost completely covered by a strong, fibrous membrane called the periosteum.

The periosteum contains many blood vessels that supply the bone with blood. Its inner surface contains cells capable of forming new bone.

In adults, all bones consist of thin sheets of bone cells. These are arranged in two ways:

In hard or compact bone, the sheets are arranged in concentric cylinders that run along the length of the bone. They enable the bone to bear stress.

In spongy or cancellous bone, the sheets are arranged as a honeycomb of irregular layers.