The mediastinum is the space in the middle of the chest between the two pleurae. It extends from the sternum to the spine, and contains all the viscera in the thorax except the lungs.

Anterior Mediastinum.—Bounded in front by the sternum, behind by the pericardium, at the sides by the pleurae. It contains the origins of the sterno-hyoid, sterno-thyroid, and the triangularis sterni muscles, internal mammary glands of the left side, remains of thymus gland, lymphatic vessels, and loose areolar tissue.

Middle Mediastinum.—Contains heart, pericardium, ascending aorta ; lower half superior vena cava, with vena azygos opening into it ; bifurcation of trachea and two bronchi, pulmonary arteries and veins, phrenic nerves, and some lymphatic glands.

Posterior Mediastinum.—Bounded in front by pericardium and roots of lungs, behind by vertebral column, at the sides by the pleurae. It contains the descending aorta, azygos veins, vagus and splanchnic nerves, oesophagus, thoracic duct, and some lymphatic glands.