Rib cage. Coloured frontal 3D computed tomography (CT) scan of a normal rib cage. Twelve pairs of ribs enclose the chest. They are attached at one end to the spine (backbone). The upper ribs are also attached to the breast bone (sternum, upper centre, in front of spine). The collar bones (clavicles) lie across the top of the rib cage and attach at one end to the breast bone and at the other ends to the shoulder bones. The shoulder blades (scapulas) and the head of the humerus (upper arm bone) are also seen, forming the shoulder joints.