Periosteum is a layer of dense connective tissue investing all bones except on the articular surfaces. It consists of outer and inner layers. The outer or fibrous layer is very dense fibrous connective tissue. The inner or osteogenic layer is more cellular and vascular, and is a kind of loose connective tissue containing blood vessels and fusiform osteogenic cells or osteoblasts near the surface of the bone. Sharpey's fibers are coarse collagen fibers that extend from the fibrous layer into the outer circumferential lamellae and serve to attach the periosteum to the bone.