Periosteum, tough, fibrous membrane that surrounds each bone. It is highly vascular and is the means by which the outer layers of the shafts and the greater part of the spongy portions of the bones are supplied with blood. It consists of an outer fibrous layer and an inner osteogenetic layer. The inner layer is very vascular and contains many protoplasmic cells called osteoblasts. Numerous experiments show that the formation of bone is essentially due to the action of the periosteum and that, by transplanting detached portions of periosteum into muscular or other tissues, bony tissue is generated in those parts. It appears to be the curative agent in the case of bone breakage. In most cases in which the periosteum has become detached as a result of a wound or of disease, the exposed bone perishes (except in the instance of the skull, which derives most of its nutrient matter from the dura mater, actually the periosteum of the inner surface of the skull). Inflammation of the periosteum results in a condition known as periostitis.