thoracic inlet, the upper opening of the thorax; inferiorly by the transverse thoracic plane. which is an imaginary plane passing from the sternal angle anteriorly to the lower border of the body of the 4th thoracic vertebra posteriorly; laterally by the pleurae; anteriorly by the manubrium of the sternum; posteriorly by the first four thoracic vertebral bodies. Contents muscles origins of the Sternohyoidei and Sternothyreoidei lower ends of the Longi colli arteries aortic arch brachiocephalic artery thoracic portions of the left common carotid and the left subclavian veins brachiocephalic veins and upper half of the superior vena cava left highest intercostal vein nerves vagus nerve cardiac nerve superficial and deep cardiac plexuses phrenic nerve left recurrent laryngeal nerve trachea with paratracheal and tracheobronchial lymph nodes esophagus thoracic duct remains of the thymus some lymph glands anterior longitudinal ligament Inferior mediastinum Anterior inferior mediastinum Is bounded: laterally by the pleurae; posteriorly by the pericardium; anteriorly by the sternum , the left transversus thoracis and the fifth, sixth, and seventh left costal cartilages. Contents A quantity of loose areolar tissue Some lymphatic vessels which ascend from the convex surface of the liver Two or three anterior mediastinal lymph nodes The small mediastinal branches of the internal thoracic artery Thymus (involuted in adults) superior and inferior sternopericardial ligaments Middle inferior mediastinum Bounded: pericardial sac – It contains the vital organs and is classified into the serous and fibrous pericardium. Contents the heart enclosed in the pericardium the ascending aorta the lower half of the superior vena cava with the azygos vein opening into it the bifurcation of the trachea and the two bronchi the pulmonary trunk dividing into its two branches the right and left pulmonary veins the phrenic nerves some bronchial lymphatic glands pericardiocophrenic vessels Posterior inferior mediastinum Is bounded: Anteriorly by (from above downwards): bifurcation of trachea; pulmonary vessels; fibrous pericardium and posterior sloping surface of diaphragm Inferiorly by the thoracic surface of the diaphragm (below); Superiorly by the transverse thoracic plane; Posteriorly by the bodies of the vertebral column from the lower border of the fifth to the twelfth thoracic vertebra (behind); Laterally by the mediastinal pleura (on either side). artery thoracic part of the descending aorta veins azygos vein the hemiazygos vein and the accessory hemiazygos vein nerves vagus nerve splanchnic nerves sympathetic chain esophagus thoracic duct some lymph glands Clinical significance The mediastinum is frequently the site of