II.

Bone Structure and Composition

Bone consists of living cells widely scattered within a nonliving material called the matrix. The matrix is formed by osteoblasts, cells that are constantly renewed in the bone. Osteoblasts make and secrete the protein collagen, which makes bones elastic so that they can give under the stresses generated by walking, lifting, and other activities. Osteoblasts also secrete mineral salts formed from calcium and phosphorus, which impart hardness so that bones do not break easily. If more bone is needed, new osteoblasts carry out the task of building it. As bone tissue matures, osteoblasts transform into osteocytes, mature bone cells that carry out daily cellular activities.

There are two main types of bone. Compact bone, which makes up most of the bone of arms and legs, is very dense and hard on the outside. The structural units of compact bone are osteons, elongated cylinders that act as weight-bearing pillars, able to withstand any mechanical stress placed on the bone. The center of each osteon contains a hollow canal that acts as a central passageway for blood vessels and nerves.

In some bones, internal to the compact bone is spongy bone, also known as cancellous bone, composed of a honeycomb network of bones called trabeculae that act as supporting beams. Spongy bone is designed to bear stress from several directions, such as that exerted on the pelvis in bending or stretching. The spaces between the trabeculae are filled with red bone marrow containing the blood vessels that nourish spongy bone. Spongy bone is found in bones of the pelvis, ribs, breastbone, vertebrae, skull, and at the ends of the arm and leg bones.

Surrounding both compact and spongy bone is a thin membrane, the periosteum. The outer layer of this membrane contains nerves and blood vessels that branch and travel into the bone. The inner layer of the periosteum consists mainly of osteoblasts.

The point where two or more bones come together is called a joint, or articulation. Different kinds of joints enable different ranges of motion. Some joints barely move, such as those between the interlocking bones of the skull. Other bones, held together by tough connective tissues called ligaments, form joints such as the hinge joint in the elbow, which permits movement in only one direction. The pivot joint between the first and second vertebrae allows the head to turn from side to side.

Intimately associated with bone is another type of connective tissue called cartilage. Cartilage is softer, more elastic, and more compressible than bone. It is found in body parts that require both stiffness and flexibility, such as the ends of bones, the tip of the nose, and the outer part of the ear.