Heart and rib cage. Coloured 3D computed tomography (CT) scan of a normal heart and rib cage seen from an oblique side view. The heart (lower left, coronary arteries seen on its surface) is in the chest cavity formed by the ribs. Twelve pairs of ribs circle the chest. They are attached at one end to the spine (backbone, down centre), and at the other end to the breast bone (sternum, upper left). The breast bone helps provide extra protection for the heart. The two clavicles (collar bones) meet at the top of the breast bone. The head of each humerus (upper arm bone) is seen at top left and right, forming part of the shoulder joints. The tip (chin) of the lower jaw bone is also at upper left.