X-ray photograph of a normal human chest showing the heart (opaque bulge) on the subject's left side. Visible also are the clavicles, or collar bones, a pair of slender bones which form the front part of the shoulder girdle & which articulate with the sternum or breastbone (central area joining left & right rib cage). The ribs (12 pairs) are attached at the back to one of the 12 thoracic vertebrae & in the front, the first seven pairs only, the true ribs, are joined to the sternum. The ribs enclose & protect the heart & the lungs (black area within).