An articular capsule surrounds each synovial joint or diarthrosis rather like a muff to keep the hands warm on a cold day. It is firmly attached to the bones on each side of the joint and so forms a closed cavity which contains those parts of the bones which make up the joint and also their articular cartilages.
Dissection of an articular capsule reveals that it is made of two layers of tissue. The outer layer is of white fibrous tissue of great strength. It is called the capsular ligament. It holds together the ends of the bones which form the joints and it supports the delicate inner layer, the synovial membrane. This membrane lines the entire inner surface of the capsular ligament, and it also covers parts of the bones.
The cells of the synovial membrane secrete a viscous (thick) yellow fluid which moistens all the structures within the articular capsule. Its purpose is to lubricate the joint and thus ensure that the surfaces move smoothly on one another with the minimum of wear.