BONE STRUCTURE
Bone has three major components, each with a specific function (Fig. 11-1):
Periosteum: This thick, fibrous vascular membrane covers most of the (cortical) bone surface.
Muscles, tendons, and ligaments connect to it. The periosteum does not cover the ends of long bones,
because you need cartilage there for joint movement.
Cortical and trabecular bone: Cortical bone is sometimes called dense or compact bone. It comprises
80% of adult bone mass. The outer portion of nearly all bones is composed of dense, strong, cortical
bone. The principal function of cortical bone is to provide strength and stability. Microscopic openings in
cortical bone allow blood vessels and nerves to pass through, providing nutrients and hormones to bone
cells.
Trabecular bone provides even more strength and structural stability. Because it is spongy and less
rigid, it acts more like a shock absorber. It has an open structure, which lightens the weight of bones.
The latticelike matrix of trabecular bone has small cavities that can be filled with marrow or connective
tissue. Trabecular bone is predominant in the ends of long bones, vertebrae, rib cage, and flat bones of
the pelvis. About 20% of adult bone mass is trabecular.
Bone marrow: Marrow is a spongy tissue that contains stem cells. These stem cells are one of two
types: hemopoietic, meaning they are capable of developing into blood cells (see Chapter 13), or