periosteum /per′i·os″tē·əm/ [Gk, peri + osteon, bone] , a thick, fibrous vascular membrane covering the bones, except at their extremities. It consists of an outer layer of collagenous tissue containing a few fat cells and an inner layer of fine elastic fibers. Periosteum is permeated with the nerves and blood vessels that innervate and nourish underlying bone. The membrane is thick and markedly vascular over young bones but thinner and less vascular in later life. Bones that lose periosteum through injury or disease usually scale or die.
