mediastinum: the part of the thoracic cavity between the lungs that contains the heart and aorta and esophagus and trachea and thymus
cavity: (anatomy) a natural hollow or sinus within the body
cardiopulmonary resuscitation: an emergency procedure consisting of external cardiac massage and artificial respiration; the first treatment for a person who has collapsed and has no pulse and has stopped breathing; attempts to restore circulation of the blood and prevent death or brain damage due to lack of oxygen