Classification and Structure of BonesBones are classified according to their shape (long, short, flat, or irregular). A long bone such as the humerus is often used to illustrate the major structures of a bone. The shaft, or longest portion of the bone, is referred to as the diaphysis, and at each end of the diaphysis lies an enlarged part called an epiphysis, which functions in articulating with other bones. The portion of each epiphysis that connects with another bone is covered by articular cartilage. The entire bone is covered by the periosteum, a fibrous tissue which allows an entrance and exit for blood vessels and nerves, and provides a site for the attachment of tendons (which connect muscles to bones) and ligaments (which attach bones to other bones). It is also involved in the formation and repair of bone tissue. The hardened part of the diaphysis consists of compact bone tissue, while the epiphyses are made up of spongy bone tissue. The compact bone of the diaphysis encloses a cylindrical space called the medullary canal, which contains blood vessels, nerves, and the bone marrow, a soft connective tissue involved in the production of blood cells.
Bone connective tissue, as we noted in Chapter 13, consists of cells called osteocytes which exist in cavities (lacunae) separated by a collagenous matrix. The matrix also contains large amounts of inorganic mineral salts, mainly in the form of hydroxyapatite (calcium phosphate), which gives bone tissue its great hardness. In the case of compact bone, the cavities are arranged in concentric circles around Haversian (osteonic) canals, which contain blood vessels to nourish the osteocytes. Osteocytes concentrically clustered around a Haversian canal form units called Haversian (osteonic) systems. Many of these units join together in an orientation that confers resistance to pressure, and make up the substance of the compact bone tissue. In spongy bone, the osteocytes are not clustered around Haversian canals, but instead rest within the spaces formed by the trabeculae, bony plates that are irregularly connected and cause spongy bone tissue to be strong but light.
Structure of a long bone, the humerus