What is the name of the fibrous sac that encloses a joint?

The name of the fibrous sac that encloses a joint is referred to as the joint capsule. This structure is critical in maintaining the integrity of the joint by providing stability and protecting the joint surfaces. The joint capsule is comprised of a dense layer of connective tissue that surrounds the joint, and it consists of two layers: the outer fibrous layer, which acts as a protective covering, and the inner synovial membrane, which produces synovial fluid to lubricate the joint.

In contrast, the synovial membrane is specifically the inner lining of the joint capsule, responsible for the secretion of synovial fluid but does not enclose the joint by itself. Articular cartilage is the smooth tissue that covers the ends of bones in synovial joints, allowing for smooth movement, but it does not serve the same protective and structural role as the joint capsule. The bursa, on the other hand, is a small, fluid-filled sac that helps reduce friction between tissues, but it is not the structure that encases the joint.

Thus, the designation of the joint capsule accurately captures the encompassing protective layer surrounding a joint, placing it as the correct answer.