70 BACTERIOLOGY 



them susceptible. Inhalation of ether increases the sus- 

 ceptibility to pneumococcic infection. 



Injury and perverted function also increase the sus- 

 ceptibility to infection. Thus, tuberculosis of the joints 

 and bones frequently follows traumatic injury, while 

 diminution of the acidity of the gastric juice increases 

 the susceptibility to cholera infection. Infection coin- 

 cidently of saprophytic germs with tetanus bacillus in- 

 creases the chances of tetanus infection. 



In some infections, as pneumonia, the susceptibility 

 is increased by a previous attack of the disease, while in 

 others immunity is conferred by an attack, as in small- 

 pox, or decreased susceptibility occurs without producing 

 complete immunity, as in diphtheria. 



The coverings of the external surface of the organism, 

 the skin, and mucous membranes present certain obsta- 

 cles to infection. Many conditions arise to destroy the 

 integrity of these coverings and thus increases the sus- 

 ceptibility to infection. 



The unbroken skin is practically impervious to germs, 

 though they doubtless do obtain entrance occasionally 

 by way of the hair-follicles and the ducts of the sweat- 

 glands. Scratches, minute abrasions, and other breaches 

 of continuity let down the bars for the entrance of germs, 

 though they are often destroyed by phagocytosis im- 

 mediately upon their entrance. 



The mucous membranes of the mouth, throat, and 

 nasal cavities normally present considerable resistance 



