A synovial membrane, or synovium, is the lining of the cavity of a synovial joint. It extends from the margins of one articular surface to those of the other. The synovial membrane is protected on the outside by a tough fibrous membrane referred to as the capsule of the joint. The membrane secretes a thick fluid, called synovial fluid, that lubricates the joint. If the joint is injured, excess fluid may accumulate and cause pain.