The thoracic vertebrae are the true "back" vertebral structures of the skeleton, providing the underlying support of the saddle, and the lumbar vertebrae of the loin provide the coupling that joins the back to the hindquarters. Integral to the back structure is the rib cage, which also provides support to the horse and rider. A complex design of bone, muscle, tendons and ligaments all work together to allow a horse to support the weight of a rider.