The mediastinum is the space between the mammalian lungs in their pleural cavities, including the structures in that space. It is a narrow dorso-ventrally extended cleft that extends from the sternum (breastbone) in front to the spinal column behind, and from the inlet of the thoracic duct (one of the main lymphatic vessels) at the top of the diaphragm at the bottom. The mediastinum contains the heart in its pericardium, the trachea (windpipe), the esophagus, the thymus gland, the major blood vessels entering and leaving the heart, lymph nodes and lymphatic vessels, and nerves (including the vagus nerve and phrenic nerve).