The purpose of respiration is the entry and exit of air to and from the lungs. In inspiration the chest cavity is enlarged and air enters,in expiration the reverse occurs. Respiratory movements may be thoracic or abdominal,or both. In thoracic inspiration the breast bone is lifted up and forward and the ribs are simultaneously elevated and come to lie more horizontally,thus the chest is increased in diameter both from side to side and from front to back. In abdominal inspiration the diaphragm contracts and flattens down on the abdominal organs,bulging out the abdominal wall and increasing the vertical height of the chest. Thus inspiration is an active process,due to muscular exertion. Expiration however is passive,the chest wall subsides,the abdominal muscles recoil and the diaphragm relaxes,and the air is driven out of the lungs.