Mediastinum

The mediastinum is the space in the middle of the thoracic cavity, bounded by the sternum in front, the thoracic vertebrae behind, the pleural sacs around each lung at the sides, the upper opening of the chest above and the diaphragm below. It is not an organ but a region packed with organs, vessels, nerves and lymph nodes in fatty connective tissue, and a corridor between the neck and the abdomen.

A horizontal plane drawn from the sternal angle, where the manubrium meets the body of the sternum, back to the level of the fourth thoracic vertebra divides it into superior and inferior parts, and the inferior part is split again into anterior, middle and posterior compartments. The superior mediastinum holds the aortic arch and its branches, the superior vena cava, the trachea, the esophagus and the thymus. The middle compartment contains the heart in its pericardium; the anterior one, just behind the sternum, holds fat, lymph nodes and part of the thymus; and the posterior one carries the descending aorta, the esophagus, the thoracic duct and the vagus nerves.

On a chest X-ray, a widened mediastinum can point to bleeding from a torn aorta. Masses in the anterior compartment include thymomas and lymphomas, and mediastinoscopy is used to sample lymph nodes when staging lung cancer.