Periosteum is a fibrous membrane investing the bones except at their articular surfaces. It consists of two layers, the outer of which is composed of course fibrous connective tissue containing few cells but numerous blood vessels and nerves.
The inner layer is more cellular and less vascular and contains many elastic fibres.
In the growing bones, the inner layer of periosteum is osteogenic and in the adult, this layer is converted into a row of flattened cells. Periosteum serves as a supporting bed for blood vessels and nerves going to bone and for anchorage of tendons and ligaments.
Some of the periosteal fibres pass into the bone, either obliquely or at right angles to the long axis of bone and are termed as perforating fibres of Sharpy.
Endosteum lines the surface of the cavities within a bone (Haversian canals) and also the surfaces of trabeculae in the marrow cavity. The cells of endosteal layer are like those of the periosteum and rest on a thin layer of connective tissue.
