In the natural resting position, the ribs slant downward allowing the sternum to fall backward toward the vertebral column. But when the rib cage is elevated, the ribs project almost directly forward, so that the sternum also moves forward, away from the spine, making the anteroposterior thickness of the chest about 20 per cent greater during maximum inspiration than during expiration. During inspiration the diaphragm becomes flat. The internal intercostal muscles contract during forceful expiration (not inspiration). During inspiration, the rib cage is elevated.