bone marrow [AS, ban + ME, marowe] , the soft, organic, spongelike material in the cavities of bones; also called medulla ossium. It is a network of blood vessels and special connective tissue fibers that hold together a composite of fat and blood-producing cells. Its chief function is to manufacture erythrocytes, leukocytes, and platelets. These blood cells normally do not enter the bloodstream until they are fully developed, so that the marrow contains cells in all stages of growth. If the body’s demand for leukocytes is increased because of infection, the marrow responds immediately by stepping up production. The same is true if more erythrocytes are necessary, as in hemorrhage or anemia. Red marrow is found in many bones of infants and children and in the spongy (cancellous) bone of the proximal epiphyses of the humerus and femur and the sternum, ribs, and vertebral bodies of adults. Fatty yellow marrow is found in the medullary cavity of most adult long bones.
