THE PLEURAL SACS 



343 



lung, and so establishes continuity with the pulmonary layer, 

 which completely encircles the lung, finally returning to the 

 posterior surface of the root (Fig. i 23). It is then carried 

 backwards to reach the vertebral column, where it becomes 

 continuous with the costal layer. 



The continuity of the costal and the diaphragmatic pleurae 

 can be demonstrated in frontal sections through the chest. 

 From the cupula pleurae, the costal layer descends on the ribs 

 to a lower level than that occupied by the lower border of the 

 lung during quiet respiration. It is then reflected on to the 



FIG. 123. Diagram of a Transverse Section through the Thorax 

 at the level of the root of the lung. The continuity of the 

 visceral and the parietal layers is demonstrated in the figure. 



upper surface of the diaphragm (p. 344), on which it passes 

 medially till the pericardium is reached. In that situation the 

 diaphragmatic layer becomes continuous with the pericardial 

 portion of the mediastinal pleura, which ascends over the peri- 

 cardium until it meets the lower border of the root of the lung, 

 where it establishes continuity with the pulmonary pleura. At 

 the upper border of the root of the lung, the pulmonary pleura 

 again becomes continuous with the mediastinal pleura, which 

 then ascends to the cupula pleurae. 



Surface Marking of the Pleural Sacs. The apex of the 

 lung extends upwards into the neck for J to i inch above the 

 level of the clavicle. It is everywhere in contact with the 



