506 THE THORAX AND VERTEBRAL COLUMN 



which form the lateral boundaries of the mediastinum are 

 termed the mediastinal pleura. 



When the arrangement of the pleura is examined in a 

 transverse section of the thorax at the level of the fourth thoracic 

 vertebra, it is seen that the costal pleura lines the inner 

 surfaces of the ribs and, anteriorly, it passes medially behind 

 the sternum to the median plane. The membrane is then 

 reflected backwards over the great vessels till it reaches the 

 vertebral column. 



If the section is made at a slightly lower level (Fig. 149), it 



Costal part of parietal pleur 



Pleural cavity N v 

 Visceral pleura 



Costal part of parietal pleura 

 Pleural cavity 

 Visceral pleura 



FIG. 149. Diagram of a Transverse Section through the Thorax, 

 at the Level of the Root of the Lung. 



passes through the root of the lung. On passing backwards 

 from the sternum, the mediastinal pleura clothes the pericardium 

 and reaches the anterior aspect of the root of the lung, on which 

 it is carried laterally to become continuous with the visceral 

 pleura. After covering the lung, the pleura is carried medially 

 on the posterior aspect of the root and again reaches the 

 pericardium, on which it passes backwards to the vertebral 

 column. 



When a frontal section is examined, it is found that, when 

 the costal pleura is traced downwards, it extends for some 

 distance below the lower border of the lung and is then reflected 

 on to the diaphragm. In this inferior extremity of the pleural 



