The mediastinum, a central chest compartment like a cavern located between the lungs, houses the heart. Immediately to the right of the heart is the aorta, the largest artery of the body. To the right of the aorta are groups of blood vessels (one artery and two companion veins) that run between the ribs to distribute blood through the body. The phrenic nerve, which sends messages to the diaphragm to breath, is visible as it crosses the heart vertically.