78 THE STORY OF THE BACTERIA. 



ing the sources of infection with tubercle 

 bacilli through the air, that everybody who 

 goes upon the street or enters a hospital or 

 a theatre is going, or is even liable, to ac- 

 quire tuberculosis. For, in the first place, the 

 infecting material, even under the worst con- 

 ditions, is enormously diluted by the circu- 

 lating air, so that the individual chances of 

 coming in contact with the dangerous material 

 are slight. In the second place, the average 

 healthy individual is not predisposed to the 

 disease at all, and could be affected only under 

 especially favorable conditions. Third, the 

 amount of infecting material is apt, in trans- 

 mission by the air, to be small, and this is a 

 condition which diminishes the chances of 

 danger from such exposure. Finally, every indi- 

 vidual has in his respiratory tubes an arrange- 

 ment of tiny cells whose free surfaces are cov- 

 ered with little hair-like processes called cilia. 

 These are ceaselessly waving to and fro, and 

 tend to sweep up and away from the lungs for- 

 eign particles which may be breathed in with 

 the air. But notwithstanding all these condi- 

 tions which serve to guard the exposed indi- 



