The mediastinal surface (facies mediastinalis; inner surface) is in contact with the mediastinal pleura.
It presents a deep concavity, the cardiac impression, which accommodates the pericardium; this is larger and deeper on the left than on the right lung, on account of the heart projecting farther to the left than to the right side of the median plane.
Above and behind this concavity is a triangular depression named the hilum,where the structures which form the root of the lung enter and leave the viscus. These structures are invested by pleura, which, below the hilus and behind the pericardial impression, forms the pulmonary ligament.
On the right lung,, immediately above the hilus, is an arched furrow which accommodates the azygos vein (Groove for azygos vein); while running upward, and then arching lateralward some little distance below the apex, is a wide groove for the superior vena cava (Groove for superior vena cava) and right innominate vein (Groove for brachiocephalic vein); behind this, and nearer the apex, is a furrow for the innominate artery. Behind the hilus and the attachment of the pulmonary ligament is a vertical groove for the esophagus (Groove for esophagus); this groove becomes less distinct below, owing to the inclination of the lower part of the esophagus to the left of the middle line. In front and to the right of the lower part of the esophageal groove is a deep concavity for the extrapericardiac portion of the thoracic part of the inferior vena cava (Groove for inferior vena cava).
On the left lung, immediately above the hilus, is a well-marked curved furrow produced by the aortic arch (Groove for aortic arch), and running upward from this toward the apex is a groove accommodating the left subclavian artery; a slight impression in front of the latter and close to the margin of the lung lodges the left innominate vein (Groove for brachiocephalic vein). Behind the hilus and pulmonary ligament is a vertical furrow produced by the descending aorta, and in front of this, near the base of the lung, the lower part of the esophagus causes a shallow impression.