What is the fibrous membrane that covers the bones, except at joint areas?

The correct answer is the periosteum. This fibrous membrane is a dense layer of vascular connective tissue that envelops the bones, providing support and nourishment. It plays a crucial role in bone health by serving as an attachment site for tendons and ligaments, as well as housing osteoblasts, which are necessary for bone growth and repair.

The periosteum is distinct in that it covers all bone surfaces except at the joints, where articular cartilage is present to facilitate smooth movement. This unique characteristic allows the periosteum to protect the bone and serve as a conduit for blood vessels and nerves that enter the bone tissue.

In contrast, other options refer to structures with different functions. The endosteum is the thin membrane lining the medullary cavity of long bones, while the synovium lines joint cavities and produces synovial fluid for lubrication. The pericardium is the fibrous sac surrounding the heart, completely unrelated to the structure of bones. Recognizing these distinctions highlights the specific role and importance of the periosteum in skeletal health and function.