84 The Story of the Bacteria 



caused alone, by exceedingly minute, rod- 

 shaped bacteria bacilli which, in one way 

 or another, gain access to the body (Fig. 10). 

 When there, if the conditions are favorable, 

 they tend to grow, and as they do so there 

 form about them little masses 

 of new tissue, which are called 

 tubercles. The most com- 

 mon seat of the disease is the 

 lungs, but it may occur in 

 FIG. IO.-TUBERCLE any part o f the body. 



The tubercles which form 

 From the sputum in the lungs are usually lim- 



of a person having ited ^ & small ft ^ t 



tuberculosis of the 



lungs. the top of the chest. But 



sometimes they are widely 

 scattered through the lung. Plate VI. shows 

 numerous tubercles the little white spots 

 in a human lung. 



Where the tubercles form the tiny air cham- 

 bers are destroyed and the lung becomes 

 solid. This is shown in Plate VII. The dark 

 mass just beneath the surface of the lung is 

 the tubercle. Around it are the air chambers 

 quite open and useful. 



