zygomatic

Structure and Composition. Bone is not uniform in structure but is composed of several layers of different materials. The outermost layer, the periosteum, is a thin, tough membrane of fibrous tissue. It gives support to the tendons that secure the muscle to the bone and also serves as a protective sheath. This membrane encloses all bones completely except at the joints where there is a layer of cartilage. Beneath the periosteum lie the dense, hard layers of bone tissue called compact bone. Its composition is fibrous rather than solid and it gives bone its resiliency. Encased within these layers is the tissue that makes up most of the volume of bone, called cancellous or spongy bone because it contains little hollows like those of a sponge. The innermost portion of the bone is a hollow cavity containing marrow. Blood vessels course through every layer of bone, carrying nutritive elements, oxygen, and other products. Bone tissue also contains a large number of nerves. The basic chemical in bone, which gives bone its hardness and strength, is calcium phosphate.

Development. Cartilage forms the major part of bone in the very young; this accounts for the great flexibility and resiliency of the infant skeleton. Gradually, calcium phosphate collects in the cartilage, and it becomes harder and more brittle. Some of the cartilage cells break loose, so that channels develop in the bone shaft. Blood vessels enter the channels, bearing with them small cells of connective tissue, some of which become osteoblasts, cells that form true bone. The osteoblasts enter the hardened cartilage, forming layers of hard, firm bone. Other cells, called osteoclasts, work to tear down old or excess bone structure, allowing the osteoblasts to rebuild with new bone. This renewal continues throughout life, although it slows down with age.

Cartilage formation and the subsequent replacement of cartilage by hard material is the mechanism by which bones grow in size. During the period of bone growth, cartilage grows over the hardened portion of bone. In time, this layer of cartilage hardens as calcium phosphate is added, and a fresh layer grows over it, and it too hardens. The process continues until the body reaches full growth. Long bones grow in length because of special cross-sectional layers of cartilage located near the flared ends of the bone. These harden and new cartilage is produced by the same process as previously described.

Bone Disorders. fracture, a break in the bone, is the most common injury to the bone; it may be closed, with no break in the skin, or open, with penetration of the skin and exposure of portions of the broken bone. osteoporosis is excessive brittleness and porosity of bone in the aged. osteomyelitis is a bone infection similar to a boil on the skin, but much more serious because blood supply to bone is less exquisite than that to other body organs and bone metabolizes more slowly, so that the infection can destroy the bone and invade other body tissues. osteomalacia is the term used for rickets when it occurs in adults. In these diseases there is softening of the bones, due to inadequate concentration of calcium or phosphorus in the body. The usual cause is deficiency of vitamin D, which is required for utilization of calcium and phosphorus by the body. In osteitis fibrosa cystica, bone is replaced by fibrous tissue because of abnormal calcium metabolism. The condition usually is due to overactivity of the parathyroid glands. osteoma refers to abnormal new growth, either benign or malignant, of the tissue of the bones. Although it is not common, it may occur in any of the bones of the body, and at any age.