THE HIP JOINT : The hip joint comprises of a long thigh bone articulated with cup shaped cavity of hip bone.Rounded head of the thigh bone fits into the cavity to from a ball and socket joint. The surface of the head of the thigh bone and the socket are covered by specialised tissue called articular cartilage. The joint cavity is lined by a membrane called the synovium which secretes synovial fluid into the joint and allows smooth movement of the ball within the socket.