THE PLEURAL SACS 345 



wards and laterally behind the seventh costal cartilage and 

 across the seventh intercostal space. In the nipple line, it 

 crosses the bony extremity of the eighth rib. It continues to 

 descend until it reaches the mid-axillary line, where it crosses 

 the tenth rib. On the posterior surface of the body, the 

 costo-diaphragmatic line of reflection ascends slightly as it 

 passes medially. It crosses the eleventh and twelfth ribs and 

 reaches the vertebral column opposite the lower border of the 

 twelfth thoracic vertebra (PI. III.). Thus the whole of the 

 lower limit of the pleural sac is placed under cover of the ribs 

 except its postero-medial corner, which descends below the 

 neck of the twelfth rib. The same line may be used to map 

 out the costo-diaphragmatic line of reflection on both sides of 

 the body. 



The lower limit of the pleural sac is placed at a much 

 lower level than the lower border of the lung during quiet 

 respiration, and throughout this area the costal and diaphrag- 

 matic pleurae are in contact with one another. On the right 

 side, the lower part of the pleural sac intervenes between the 

 liver and the surface of the body, and, in consequence, it may 

 be difficult to determine the presence of a small effusion in 

 this situation. On the left side, this part of the pleura over- 

 lies the stomach anteriorly, and, when healthy, it does not 

 cause any alteration in the tympanitic note obtained on 

 percussion over that viscus. Effusions of fluid, however 

 small, which gravitate downwards into this part of the left 

 pleural sac, can readily be detected, since they encroach on 

 Traube's space (p. 244) from above, and an area of dulness to 

 percussion is found to intervene between the lung resonance 

 above and the stomach tympanitis below. Posteriorly, on the 

 left side, the pleural sac completely overlies the spleen, which 

 may be thrust downwards from under cover of the ribs by 

 large pleural effusions on the left side. 



Pleuritic Effusions. In health, there is a constant circula- 

 tion of lymph through the pleural sac. The lymph enters the 

 sac by a process of transudation from the neighbouring blood- 



