358 DISEASES OF THE RESPIRATORY ORGANS. 



Microscopically the serous exudate is found to contain a few 

 white and red corpuscles and desquamated endothelial cells. 

 In chemical composition and physical appearances it resembles 

 blood-serum. When the effusion is excessive the lung is 

 compressed against the vertebral column, airless and almost 

 bloodless. 



Empyema: A purulent effusion into the pleural cavity 

 results either primarily from the presence of some pus-pro- 

 ducing micro-organism, or from secondary infection of a simple 

 serous effusion. The micro-organisms most frequently found 

 are the pneumococcus, streptococcus, staphylococcusand tuber- 

 cle bacillus; occasionally the bacillus coli, typhoid and others. 

 The pleura is greatly thickened, and on its surface are more 

 or less abundant granulations. A localized necrosis may 

 result in perforation of the pleura and discharge of the effu- 

 sion externally through the thoracic wall, into the lungs, or 

 through the diaphragm into the peritoneal cavity. 



