The sternum is a long, flat bone, forming the middle portion of the front of the chest. The top of the sternum supports the clavicles (collarbones) and its edges join with the cartilages of the first seven pairs of ribs.The inner surface of the sternum is also the attachment of the sternopericardial ligaments.he sternum is a long, flat bone, forming the middle portion of the front of the chest. The top of the sternum supports the clavicles (collarbones) and its edges join with the cartilages of the first seven pairs of ribs.