![]() ![]() osteoporosis is excessive brittleness and porosity of bone in the aged. ![]() 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. These harden and new cartilage is produced by the same process as previously described.īone Disorders. Long bones grow in length because of special cross-sectional layers of cartilage located near the flared ends of the bone. The process continues until the body reaches full growth. In time, this layer of cartilage hardens as calcium phosphate is added, and a fresh layer grows over it, and it too hardens. During the period of bone growth, cartilage grows over the hardened portion of bone. This renewal continues throughout life, although it slows down with age.Ĭartilage formation and the subsequent replacement of cartilage by hard material is the mechanism by which bones grow in size. Other cells, called osteoclasts, work to tear down old or excess bone structure, allowing the osteoblasts to rebuild with new bone. The osteoblasts enter the hardened cartilage, forming layers of hard, firm bone. Blood vessels enter the channels, bearing with them small cells of connective tissue, some of which become osteoblasts, cells that form true bone. Some of the cartilage cells break loose, so that channels develop in the bone shaft. Gradually, calcium phosphate collects in the cartilage, and it becomes harder and more brittle. Cartilage forms the major part of bone in the very young this accounts for the great flexibility and resiliency of the infant skeleton. The basic chemical in bone, which gives bone its hardness and strength, is calcium phosphate.ĭevelopment. Bone tissue also contains a large number of nerves. Blood vessels course through every layer of bone, carrying nutritive elements, oxygen, and other products. The innermost portion of the bone is a hollow cavity containing marrow. 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. Its composition is fibrous rather than solid and it gives bone its resiliency. Beneath the periosteum lie the dense, hard layers of bone tissue called compact bone. This membrane encloses all bones completely except at the joints where there is a layer of cartilage. It gives support to the tendons that secure the muscle to the bone and also serves as a protective sheath. The outermost layer, the periosteum, is a thin, tough membrane of fibrous tissue. Bone is not uniform in structure but is composed of several layers of different materials. The bones themselves act as a storehouse of calcium, which must be maintained at a certain level in the blood for the body's normal chemical functioning. The bone marrow in the center manufactures blood cells. Other bones, such as the jaw and the bones of the fingers, are concerned chiefly with movement. Some, such as the pelvis, are mainly supporting structures. ![]() An example is the skull, which encloses the brain, the back of the eyeball, and the inner ear. Some bones have a chiefly protective function. Bones, ligaments, muscles, and tendons are the tissues of the body responsible for supporting and moving the body. Collectively they form the skeletal system, a structure bound together by ligaments at the joints and set in motion by the muscles, which are secured to the bones by means of tendons. There are 206 separate bones in the human body. See anatomic Table of Bones in the Appendices for regional and alphabetical listings of bones, and see color plates 1 and 2. any distinct piece of the skeleton of the body.
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