text: Bone consists of a chemical mixture of inorganic salts (65 to 70 per cent) and various organic substances (30 to 35 per cent) and is both hard and elastic. Its hardness is derived from inorganic constituents, principally calcium phosphate and calcium carbonate, with small amounts of fluorides, sulphates, and chlorides; its elasticity is derived from such organic substances as gelatin, collagen, and traces of elastin, cellular material, and fats. Internal tubular structures called Haversian canals contain nerve tissues and blood vessels that provide bones with organic nourishment. Surrounding these canals is a somewhat porous tissue composed of thin plates, known as lamellae, and usually containing cavities filled with a network of connective tissue called marrow or myeloid tissue. Bone marrow accounts for from 2 to 5 per cent of the body weight of a person and consists of tissue of two types. Yellow bone marrow is made up principally of fat, and red bone marrow is tissue in which red and white blood cells and blood platelets originate. The external portions of bones, enclosing all the components mentioned above, include the compact and hardest of all bone tissue, which is in turn generally sheathed by a vascular, fibrous membrane known as the periosteum.