Light micrograph of synovial joint, fetal phalanges. A synovial joint joins bones with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulating surfaces. They are the most common and most movable type of joint in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones. Phalanges are digital bones in the hands and feet of most vertebrates. Magnification unknown.