Which structure is a dense, fibrous membrane that covers long bones except at the joint surfaces?

The periosteum is a dense, fibrous membrane that plays a vital role in the structure and function of long bones. It covers the entire surface of these bones, except at the joints where the articular cartilage is present. This membrane serves several important functions: it provides a protective layer, contributes to the attachment of muscles and tendons through its connective tissue fibers, and contains blood vessels and nerves that are essential for bone health and maintenance.

Furthermore, the periosteum is instrumental in the process of bone growth and repair. Its inner layer contains osteoblasts, which are cells responsible for the formation of new bone tissue. This feature is particularly important in the healing of fractures, as the periosteum assists in the regeneration of bone.

In contrast, other structures mentioned do not fulfill this specific role. The endosteum lines the medullary cavity and is involved in the growth and repair of bone from the inside, but it does not cover the outer surface. Articular cartilage is found at the joints, providing a smooth surface for movement but does not cover the entire bone surface, and the medullary cavity is the hollow region within long bones that contains bone marrow, not a protective outer membrane. Thus, the periosteum is