Hip joint, artwork. This is a ball and socket joint. The socket is a cup-shaped cavity in the pelvic bone (upper left), which the ball (centre left), at the head of the femur (thigh bone, right) fits in to. The joint is encased by the synovial membrane (purple) and bathed in synovial fluid (blue). Cartilage, whichc protects the bone from wear and tear, is red. This type of joint provides the mobility needed for walking and running.