The ribs are attached by the transverse processes to the dorsal vertebræ, and in front by cartilages to the sternum or breast-bone. They are twelve in number on each side. The interstices of this bony cage are filled with muscles; they cover and form with it the walls of the chest, called also the thorax or thoracic cavity, which contains the lungs and heart The flexibility of the costal cartilages, and the mobility of the articulations of the ribs with the spinal column, allow the thorax to expand and to contract in respiration.