Major contents of mediastinum:
Superior : trachea, esophagus , thoracic duct , thymus , aortic arch , left common carotid , left subclavian , brachiocephalic trunk , rt and left brachiocephalic veins , SVC (upper portion), Azygos vein (near its opening in SVC) , left recurrent laryngeal nerve.
Anterior: thymus , mediastinal branches of internal thoracic vessels.
Middle: Heart , Phrenic nerve, tracheal bifurcation , ascending aorta , SVC , IVC, pulmonary trunk
Posterior: thoracic duct, thoracic aorta, esophagus, vagus nerve, Azygos, Hemiazygos vein.