TOPOGRAPHY OF THE THORAX AND ABDOMEN I9 



stinum, the third, posterior to the hilus, corresponding to 

 the posterior mediastinum, and the fourth, anterior to the 

 hilus, corresponding to the middle and anterior media- 

 stinum. 



The hilus is an area roughly oval in outline, much 

 nearer the posterior than the anterior part of the middle 

 of the inner surface of the lung, through which the root 

 structures pass from the mediastinum to the lung. The 

 root of the right lung begins above at a plane passed 

 through the first intercostal space and the lower part of the 

 fourth thoracic vertebra, and ends below at a plane passed 

 through the upper part of the fourth sternochondral articu- 

 lation and the lower border of the seventh thoracic verte- 

 bra. It lies between the vena cava superior and right auri- 

 cle anteriorly and the vena azygos major and oesophagus 

 posteriorly. The arrangement of the three main structures 

 forming the root, from above downward is bronchus, artery, 

 vein. The artery lies in a plane anterior to the bronchus 

 and directly over the vein. The vena azygos major arches 

 over the bronchus and empties into the vena cava superior 

 opposite the middle of the fourth thoracic vertebra. The 

 root is covered with pleura reflected from the mediastinum 

 onto the inner surface of the lung. Anteriorly the reflection 

 is from the vena cava superior to the pulmonary artery 

 above and from the pericardium, at the level of the auri- 

 cula, to the pulmonary vein below. Superiorly the pleura 

 passes from the side of the trachea over the right bronchus 

 onto the lung. The arch of the vena azygos major lies 

 directly under the pleura covering this part of the root. 

 Posteriorly the reflection is from the vena azygos major, 

 except in the uppermost part, where that vein lies in the 

 posterior part of the root. Here the reflection is from the 

 side of the oesophagus. Inferiorly the pleura covering the 



