Parts of a Long Bone

A long bone, such as one in an arm or leg, has at its end an expanded portion called an "epiphysis," which articulates or forms a joint with another bone. On its outer surface, the articulating portion of the epiphysis is coated with a layer of hyaline cartilage called "articular cartilage." This layer is resistant to wear and produces a minimum of friction when it is compressed as the joint is moved. The shaft of the bone, which is located between the epiphyses, is called the "diasphysis." Except for the articular cartilage on its ends, the bone is completely enclosed by a tough, vascular covering of fibrous tissue called the "periosteum." This membrane is firmly attached to the bone, and its fibers are continuous with various ligaments and tendons that are connected to it. It also functions in the formation and repair of bone tissue. The wall of the disphysis is composed of tightly packed tissue called "compact bone." This type of bone is solid, strong, and resistant to bending. The epiphysis, on the other hand, are composed of largely "spongy" (cancellous) bone with thin layers of compact bone on their surfaces. Spongy bone provides strength and its bony plates are most highly developed in the regions of the epiphyses that are subjected to forces of compression. Both compact and spongy tissues are usually present in every bone. Compact bone in the diaphysis of a long bone forms a rigid tube with a hollow chamber called the "medullary cavity." The cavity is continuous with the spaces of the spongy bones.