Respiration is process of taking air into and expelling air out of the lungs. Taking air into the lungs is called inspiration (i.e.,inhalation); expelling air from the lungs is called expiration (i.e.,expiration). When air is taken into the lungs, the volume of the thoracic cavity expands; when air is expelled from the lungs, the volume of the thoracic cavity decreases. Therefore any Muscle that has the ability to change the Volume of the thoracic cavity is a muscle of respiration. Generally the volume of the thoracic cavity can be affected in two ways.