The mediastinum is that part of the thoracic cavity located centrally between the pleural cavities. It extends anteroposteriorly from the inner aspect of the sternum to the spine and superoinferi-orly from the thoracic inlet to the diaphragm. It can be subdivided arbitrarily into anterior, superior, middle and posterior compartments. (Figure 40.1).

The anterosuperior compartment contains the thymus gland, lymph nodes, vessels and fat. The thymus is large at birth but atrophies after puberty and in the adult is variable in size. It can extend down beyond the aortic arch and lie in front of the brachiocephalic veins and left common carotid artery. Parathyroid tissue may be embedded in it. The great vessels, the aorta, the superior vena cava and the azygos vein lie in the anterosuperior compartment.

The middle compartment contains the heart, pericardium, trachea, major bronchi, the pulmonary vessels, phrenic and vagus nerves.

The posterior (paravertebral) compartment contains the sympathetic chain, vagus nerves, oesophagus, thoracic duct, descending aorta, azygous and hemiazygous veins and lymph nodes.

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