Each human lung is in the rib cage bounded below by the diaphragm. Breathing works by making the rib cage larger, decreasing the pressure in the lungs, and air is sucked in (inspiration). Expelling depleted air is the reverse, where the cage collapses and the lung pressure is increased. The breathing action is driven by the contraction and relaxation of the diaphragm. When the diaphragm muscle contracts, it moves downward, making the chest or thoracic cavity longer, while its associated intercostal muscles contract, widening the chest, allowing inspiration. The diaphragm and external intercostal muscles then relax and expiration occurs. The diaphragm is now back in its resting stage due to elastic recoil and the pull of the abdominal muscles.