The parietal pleura surrounding the root of the lung extends downward beyond the root of the lung as a fold called pulmonary ligament. The root of the lung is covered in front, above, and behind by pleura; at its inferior border, the investing layers come into contact. Here they form a sort of mesenteric fold, the pulmonary ligament, which extends between the inferior part of the mediastinal surface of the lung and the pericardium. The pulmonary ligament droops down from the hilum of the lung.
Just above the diaphragm the ligament ends in a free falciform border. It serves to retain the inferior part of the lung in position.