THORACIC CAVITY 53 



may be a groove caused by the vena azygos (Fig. 18). The 

 left lung, in the corresponding situation, is marked by a 

 deep longitudinal groove which is produced by the contact 

 of the lung with the descending thoracic aorta ; and, close 

 to the base, a small triangular area, anterior to the aortic 

 groove, lies in relation with the left border of the lowest part 

 of the thoracic portion of the oesophagus (Fig. 19). 



The portion of the mediastinal surface which lies above 

 the hilum and pericardial hollow is applied to the lateral 

 aspect of the superior mediastinum and the markings upon 

 it are different on the two sides. On the left side, a broad 

 deep groove, caused by the aortic arch, curves over the hilum 

 and becomes continuous posteriorly with the aortic groove 

 on the posterior mediastinal area (Figs. 18, 20, 21). From 

 this arched groove a sharply cut sulcus, caused by the left 

 subclavian artery, ascends on the medial side of the apex 

 (Figs. 1 8, 22), and, turning laterally above, it crosses the 

 anterior border of the apex a short distance below the 

 summit. Immediately anterior to the subclavian sulcus 

 the medial surface of the apex is occasionally marked by a 

 shallow sulcus caused by the left innominate vein, and more 

 inferiorly its anterior margin is depressed by the first rib. 

 That portion of the surface which lies posterior to the sub- 

 clavian sulcus is separated by areolar tissue from the 

 oesophagus and thoracic duct. 



On the right lung also a curved sulcus arches over the 

 hilum. It is caused by the vena azygos, as it passes forwards 

 to join the superior vena cava. The groove for the vena 

 azygos is much narrower than the sulcus on the left lung due 

 to the aortic arch. From the anterior end of the sulcus for 

 the azygos vein a broad shallow sulcus passes upwards to 

 the lower and anterior part of the apex ; it is produced by 

 the superior vena cava and the right innominate vein 

 (Figs. 19, 20, 22), and in some cases it is prolonged to the 

 upper part of the apex by a slight longitudinal depression 

 due to the pressure of the internal jugular vein. Arching 

 laterally, across the upper part of the anterior aspect of the 

 apex, there is a shallow groove produced by the right sub- 

 clavian artery. Posterior to the sulcus for the innominate 

 vein, the medial surface of the apex lies in relation with the 

 right side of the trachea and the vagus, and still further 

 posteriorly it is either in relation with the right border of the 



