What is the thin, tough membrane that covers a bone?

The thin, tough membrane that covers bones is known as the periosteum. It typically serves as a protective layer for all areas of a bone, except where cartilage is present.

The periosteum is a type of connective tissue that is several cell layers thick. It serves to aid in muscle, ligament and tendon attachment to the bone while providing a portion of its blood supply to help ensure nourishment of the bone. The periosteum is made up of an outer fibrous layer and an inner osteogenic layer. The fibrous layer contains fibroblasts, which are essential to the bone's healing process and protection, while the osteogenic layer contains osteoblasts, which are responsible for proper bone growth.