Periosteum(redirected from periosteum externum)
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connective tissue in vertebrate animals and man that invests bone (excluding the articular facets, linea aspera, and tuberosities); it joins the bone by means of bundles of collagen fibers and blood vessels.
Three layers are distinguished within the periosteum: the external layer (adventitia), which consists of fibrous connective tissue rich in blood vessels; the middle fibroelastic layer, which consists of collagen fibers; and the internal osteoblastic layer, which contains osteoblasts and is usually absent in adult organisms. The periosteum joins bone with surrounding tissues, assures growth of bone tissue (mainly in thickness), and in adult organisms assures regeneration of bone tissue.