The left phrenic nerve enters the superior mediastinum in a position similar to the path taken by the right phrenic nerve. It lies lateral to the left vagus nerve and lateral and slightly posterior to the beginning of the left brachiocephalic vein (see Fig. 3.89), and continues to descend across the left lateral surface of the arch of the aorta, passing superficially to the left vagus nerve and the left superior intercostal vein.