A fresh or living bone is covered with a delicate, tough, fibrous membrane, called the periosteum. It adheres very closely to the bone, and covers every part except at the joints and where it is protected with cartilage. The periosteum is richly supplied with blood-vessels, and plays a chief part in the growth, formation, and repair of bone. If a portion of the periosteum be detached by injury or disease, there is risk that a layer of the subjacent bone will lose its vitality and be cast off.[5] 30. Microscopic Structure of Bone. If a very thin slice of bone be cut from the compact tissue and examined under a microscope, numerous minute openings are seen. Around these are arranged rings of bone, with little black bodies in them, from which radiate fine, dark lines. These openings are sections of canals called _Haversian canals_, after Havers, an English physician, who first discovered them. The black bodies are minute cavities called _lacunæ_, while the fine lines are very minute canals, _canaliculi_, which connect the lacunæ and the Haversian canals. These Haversian canals are supplied with tiny blood-vessels, while the lacunæ contain bone cells. Very fine branches from these cells pass into the canaliculi. The Haversian canals run lengthwise of the bone; hence if the bone be divided longitudinally these canals will be opened along their length (Fig. 13). Thus bones are not dry, lifeless substances, but are the very type of activity and change. In life they are richly supplied with blood from the nutrient artery and from the periosteum, by an endless network of nourishing canals throughout their whole structure. Bone has, therefore, like all other living structures, a _self-formative_ power, and draws from the blood the materials for its own nutrition. Illustration: Fig. 13. A, longitudinal section of bone, by which the Haversian canals are seen branching and communicating with one another; B, cross section of a very thin slice of bone, magnified about 300 diameters—little openings (Haversian canals) are seen, and around them are ranged rings of bones with little black bodies (lacunæ), from which branch out fine dark lines (canaliculi); C, a bone cell, highly magnified, lying in lacuna. The Bones of the Head. 31. The Head, or Skull. The bones of the skeleton, the bony framework of our bodies, may be divided into those of the head, the trunk, and the limbs. The bones of the head are described in two parts,—those of the cranium, or brain-case, and those of the face. Taken together, they form the skull. The head is usually said to contain 22 bones, of which 8 belong to the cranium and 14 to the face. In early childhood, the bones of the head are separate to allow the brain to expand; but as we grow older they gradually unite, the better to protect the delicate brain tissue. 32. The Cranium. The cranium is a dome-like structure, made up in the adult of 8 distinct bones firmly locked together. These bones are: