326 CLINICAL APPLIED ANATOMY. 



The pleural sacs in health are potential only, the parietal and 

 visceral layers being in apposition. When roughened by inflam- 

 mation the gliding of one layer over the other, which accompanies 

 the respiratory movements, causes a friction sound. The 

 accumulation of fluid between the layers will obliterate the 

 sound by separating the rough surfaces. The normal proximity 

 of the layers accounts for the ease with which pleural adhesions 

 form, for the respiratory movements are lessened or absent over 

 the inflamed areas. The inflammation of apical pleurisy may 

 extend to the surrounding subpleural connective tissue and cause 

 compression of the vagus, phrenic or sympathetic nerves which 

 are in intimate relation with the pleural domes. The 

 association of mediastinitis with pleurisy has already been 

 referred to. 



The narrowness of that part of the pleural cavity which 

 extends between the diaphragm and the chest wall renders it 

 inadvisable to make the incision for drainage of an empyema, 

 in the lower part of the sac. After the pus has been evacuated, 

 the diaphragm rises and approaches the chest wall, and so tends 

 to block up the opening. Incisions in the scapular line, with 

 resection of a part of the ninth rib, are free from this objection 

 and afford efficient drainage of the sac in recumbency. The 

 seventh or eighth ribs are sometimes resected, but although they 

 are uncovered in this position when the arm is elevated, they 

 are overlapped by the angle of the scapula when the arm is 

 lowered. For this reason, and 011 account of the presence of 

 the latissimus dorsi, some prefer to make the incision a little 

 more forward. 



Empyemata, if left alone, may open into a bronchus, and the 

 danger of this undesirable occurrence has been much exaggerated. 

 They may also point externally. The weakest part of the chest 

 wall is in the interchondral spaces, where the intercostal muscles 

 are deficient and the anterior perforating branches of the 

 internal mammary afford a track. But the actual places at 

 which a neglected empyema may present are various. Some- 

 times it presents near the nipple, sometimes in the lumbar 



