A synovial joint is surrounded by a two layer joint capsule which encloses a fluid filled space called the synovial cavity. The outer layer of this structure consists of dense connective tissue which is continuous with the periosteum of each bone. The outer layer may be reinforced by ligaments which are bundles of collagenous fibers. Some of these bundles are part of the capsule while other are located outside of the capsule. By definition, the structures, such as some ligaments, that are located outside the capsule are called accessory structures. The inner layer of the joint capsule is the synovial membrane, consisting of loose connective tissue. The inner lining of the joint capsule secretes a fluid called synovial fluid. This fluid is viscous, and moistens and lubricates the surfaces within the joint capsule