chapter  2
Pages 42

An azygos fissure is an incidental finding of little consequence, formed by a laterally displaced azygos vein. The vein is enveloped by two layers of visceral and parietal pleura on each side (so there are four layers of pleura in total), giving the appearance of a relatively dense azygos fissure on a PA radiograph. The right lung medial to the fissure is termed an azygos lobe. Approximately 1% of the population have an azygos lobe.