Hip joints. Coloured X-ray of a woman's groin and upper legs, revealing the bones that form her hip joints. The left hip (identical to the right one) is at upper left in this rear view. It is a ball- and-socket joint that provides the mobility needed for walking and running. The socket is a cavity in a bone of the pelvis, a set of bones that join to the spine in the main body torso. The ball of the hip joint is the head of the femur (thigh bone). This head is seen joined by a neck to the rest of the femur, the long bone descending down left. The same structures form the right hip joint. The base of the spine is at top centre, and the distinctive loops at the base of the pelvis are also seen.