346 THE RESPIRATORY SYSTEM 



vessels and it leaves the sac through small stomata, which are 

 found in both the pulmonary and parietal pleurae, and which 

 lead into small lymph vessels. Lymph, therefore, may pass 

 from the pleural sac into the superficial lymph vessels of the 

 lung and so reach the lymph glands at the hilus (p. 353), 

 or it may enter the lymph vessels in the thoracic wall and 

 so reach the internal mammary lymph glands. When the 

 pleural membrane becomes inflamed, there is an increased 

 flow of lymph into the sac, and at the same time the stomata 

 may become obstructed by fibrinous threads. It has been 

 suggested (West) that lymph only leaves the pleural sac 

 during expiration, and, therefore, large effusions, which com- 

 press the lung so as to diminish its movement to a marked 

 degree, remain unabsorbed. This view gains support from 

 the fact that the removal of a part only of a large effusion is 

 followed by re-absorption of the remainder, as it enables the 

 excursions of the lung to be increased. 



When the upper limit of a large pleural effusion is 

 determined by percussion, it is very rarely found to be 

 horizontal. Garland and others state that the upper border 

 of the dull area forms a curve which is convex upwards, the 

 summit of the curve being placed at some little distance from 

 the posterior median line. Sahli, however, claims that, when 

 light percussion is employed, it is possible to demonstrate that 

 the upper border of the dull area is a horizontal line on the 

 posterior aspect of the body but that it inclines downwards 

 when traced forwards round the chest. He lays stress on the 

 necessity for light percussion near the posterior median line, 

 as it is only when that method is employed that the observer 

 can avoid the alteration in the percussion note caused by the 

 healthy lung of the opposite side. The same author suggests 

 that the fluid spreads upwards more easily in the posterior 

 part of the pleural sac as the posterior part of the lung, being 

 more voluminous, possesses a greater retractive power than the 

 anterior part. 



Exploratory needling of the pleural sac is carried out 



