bone, hard tissue that forms the skeleton of the body in vertebrate animals. In the very young, the skeleton is composed largely of cartilage and is therefore pliable, reducing the incidence of bone fracture and breakage in childhood. The inorganic, or mineral, content of bone is mainly calcium, phosphate and carbonate minerals. The organic content is a gelatinous material called collagen. As the body grows older, decreases in bone mass may lead to an increased vulnerability to fractures. Bone fractures heal naturally, although they are often aided through restriction of movement in the affected area. Bones assume a variety of sizes and shapes; however, all bone tissue has a three-layered composition. A spongy layer forms the interior. Long bones (such as those in the arms and legs) are hollow, the inner spaces being filled with marrow (see bone marrow), important in the formation of blood cells. Surrounding the spongy, inner layer is a hard, compact layer that functions as the basic supportive tissue of the body. The outer layer is a tough membrane called the periosteum, which sheaths most bones. Although bone appears solid, it contains numerous microscopic canals permitting the passage of blood vessels and nerve fibers. Two types of bone are present in most bones: compact, which constitutes the shaft, and cancellous, an extremely strong variety which makes up the enlarged ends of the bone. See also osteoporosis.