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The structure of a bone may be studies by sectioning a long bone, for example the femur, or thigh bone. The outer layer of solid or compact bone is called the cortex. It is thickest at the shaft, or diaphysis, of the bone, and thinnest at the ends, or epiphyses. Close examination of compact bone shows that the matrix is laid down in layers (or lamellae) which are closely apposed to each other (similar in some respects to the layers of an onion). The outer surface of the cortex is normally smooth, except at the sites of attachment of muscles or tendons where it may be either raised or depressed. Within the epiphyses, there is predominantly cancellous or spongy bone, in which the matrix is laid down as trabeculae, or struts, in a three-dimensional interlacing pattern, with distinct spaces between them. In the centre of the shaft is the medullary cavity which contains bone marrow. In young animals this contains red marrow which produces red and white blood cells. With ageing this is partially replaced by yellow marrow which contains predominantly fat.
A feature of bone is its ability to repair defects or fractures by producing new bone tissue, rather than forming a scar of fibrous tissue, as occurs in most of the other organs of the body. Moreover, because it is a living tissue, it can re-model according its shape as the limb returns to use, though it will not become normal. Prolonged recumbency causes a loss of bone mass-a result of disuse.
The outer surface of the bone is covered by a fibrous membrane, the periosteum. This is not only the limiting outer membrane of the bone, but it also provides blood to the bone and contains cells which can produce new bone tissue. This is illustrated by the periosteum helping to form the bridging callus at a healing fracture. The periosteum is also very sensitive to pain – a kick on the shins is so painful because there is very little natural padding in humans between the skin and the periosteum at this site. At the epiphyses, the periosteum is continuous with a layer of articular or hyaline cartilage, which acts as the articular surface of the joint. Cartilage does not normally contain calcium, so is radiolucent, and appears dark on radiographs.
In the shaft of a long bone there is at least one small opening in the cortex, this is the nutrient foramen. The nutrient artery of the bone runs through this into the medullary cavity, where it divides to supply the bone tissue.
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