The central part of the thorax between the two pleural cavities contains a group of structures collectively termed the mediastinum. These include the heart and great vessels, the trachea and the oesophagus. The mediastinum extends from the superior thoracic aperture above to the diaphragm below and from the sternum in front to the thoracic vertebral bodies behind (Fig. 2.28). By convention, the mediastinum is divided into superior and inferior parts by an imaginary horizontal plane passing through the manubriosternal joint and the lower part of the fourth thoracic vertebra. The superior mediastinum lies between this plane and the superior thoracic aperture and contains the superior vena cava and its tributaries, the arch of the aorta and its branches and the trachea. Also passing through this region are the oesophagus, the thoracic duct and the right and left vagus and phrenic nerves.
The inferior mediastinum lies between the imaginary plane and the diaphragm and consists of three compartments. The largest of these is the middle mediastinum, containing the heart and its covering of fibrous pericardium. In front of the middle mediastinum lies the anterior mediastinum, consisting of a small amount of fat and the remnants of the thymus gland. Behind the fibrous pericardium lies the posterior mediastinum, traversed by the descending thoracic aorta, the oesophagus, the thoracic duct and the azygos venous system. The sympathetic trunks run alongside the thoracic vertebral bodies.