Breathing comprises of inspiration and expiration of air.
1. Inspiration: During inspiration the volume of the thorax is increased by two movements, for intake of air.
(a) The muscles of the diaphragm contract and becomes flattenned.
(b) The lower ribs are raised upward and outwards by the contraction of the intercostal muscles which run obliquely from one rib to the next rib. The chest cavity enlarges, the air pressure in the lungs is decreased, air rushes into the lungs.
2. Expiration: Breathing out of air, results mainly from relaxation of muscles of the ribs and diaphragm. The chest cavity is reduced and air is forced outward through the nose and trachea.