PLEUEAL EFFUSION. 325 



affected side, the lung is probably retracted only in the neigh- 

 bourhood of the fluid, and the position of the dulness does not 

 shift appreciably with alterations in the posture of the patient. 

 The root of the lung lies opposite the fifth, sixth and seventh 

 dorsal vertebrae, and it is at this level, near the vertebral 

 column, that the signs of retracted lung may sometimes be 

 obtained in large pleural effusions. Only when the amount of 

 fluid is very considerable will actual displacement of the liver or 

 spleen occur. 



A knowledge of the surface markings of the lung fissures (see 

 pneumonia) is essential for the diagnosis of fluid collections, 

 usually purulent, between the lobes. Such collections give rise to 

 abnormal signs in the lobes which adjoin the fissures, and varia- 

 tions in these signs, such as shifting from one lobe to the other 

 and back again, should arouse suspicions of an interlobar 

 empyema. 



The dome-like projection of the abdominal space into the base 

 of the thorax allows the peritoneal cavity to invaginate the floor 

 of the chest, the lungs and pleurae fitting like a cap on the 

 convexity of the projection. Hence peritoneal friction, produced 

 between the liver or other abdominal viscus and the diaphragm, 

 may be heard over areas which are in direct surface relation with 

 the pleural sacs, for the latter extend downwards between the 

 parietes and the peritoneum. An aspirator needle, too, may 

 inadvertently be passed right through the pleural sac and with- 

 draw fluid from the peritoneal cavity. Moreover, the action of 

 the diaphragm will impart a respiratory rhythm to the peritoneal 

 friction. Any of these occurrences may give rise to difficulty in 

 diagnosis. The intimate relation of the pericardial to the 

 pleural sacs, the former being clothed laterally by the mediastinal 

 pleura and overlapped by the costo-mediastinal reflections in 

 front, explains how inflammation of one sac may easily extend 

 to the other, and also accounts for pleural friction with a cardiac 

 rhythm. A large pericardial effusion, with retraction of the lung, 

 may easily simulate pleurisy, and even be tapped without 

 discovery of the error. 



