The thoracic duct, which is the major lymphatic vessel in the body, passes through the posterior portion of the superior mediastinum (see Figs. 3.87 and 3.92). It: enters the superior mediastinum inferiorly, slightly to the left of the midline, having moved to this position just before leaving the posterior mediastinum opposite vertebral level TIV/V; and continues through the superior mediastinum, posterior to the arch of the aorta, and the initial portion of the left subclavian artery, between the esophagus and the left mediastinal part of the parietal pleura.