Glossary
periosteum
The periosteum is the tough membrane wrapped around a bone's outer surface, carrying its blood vessels and the cells that widen and mend it.
The periosteum is the tough fibrous membrane that covers the outer surface of a bone, everywhere except where cartilage covers a joint. It is not decoration. It carries the blood vessels and nerves that reach the bone, and its inner layer holds the cells that widen a growing bone and lay down the new bone that closes a fracture. Bone heals from the outside in, and that outer callus is periosteum work; a fracture in which the membrane is stripped away heals slowly or not at all. Tendons and ligaments blend into it, which is how a muscle's pull reaches the bone at all. Because it is richly supplied with nerve endings, it is also why a bone injury hurts so sharply: the bone itself carries fewer nerves than the membrane wrapped around it.
Taught in
Related terms
- diaphysis — The diaphysis is the long shaft of a long bone — a tube of compact bone wrapped around a marrow cavity.
- osteoblast — An osteoblast is the cell that builds bone: it lays down the new matrix and starts the mineral deposit that hardens it.
- epiphysis — The epiphysis is the rounded end of a long bone — spongy bone under a thin shell, shaped to meet the joint it forms.
Source: The anatomy and physiology guide in this course, whose organ and tissue descriptions follow the US National Cancer Institute's SEER Training Modules and MedlinePlus consumer health pages. Both are US federal government material, not subject to copyright in the United States. The definition above is written for this course in our own words. (SEER Training Modules accessed 2026-10-26; definition written 2026-10-27)