ESSAYS ON BAC rSEIOLOGY. 155 



trance and growth of bacteria. The broken skin not 

 only more readily admits bacteria, but the injured tis- 

 sue less easily resists their growth. 



The poison of measles does not produce either 

 whooping cough or tuberculosis, but the respiratory 

 mucous membranes, altered and damaged by measles, 

 fall a more ready prey to the next invaders. 



The inhalation of metallic or other dust cannot of 

 itself produce more than a simple inflammation with 

 resulting fibrosis. But it may open the way for the 

 entrance and growth of tubercle bacilli. 



Some of the most beautiful of bacteriological ex- 

 periments have been made to test this matter of local 

 lessened resistance. It may be interesting to quote a 

 few of these experiments. 



Bacteria injected into the circulation do not often 

 attach themselves to the heart valves, neither does a 

 direct aseptic injury of the valves, as by a clean 

 needle, produce ulcerative endocarditis. In other 

 words, simple injury, or the mere presence of bacteria, 

 do not necessarily result in ulcerative endocarditis. 

 But let the valves be deliberately injured, and about 

 ike same time let suppurative bacteria be introduced 

 into the blood current, and these bacteria will be 

 found lodged and growing at the point of lessened 

 resistance. A blow over the tibia without break of 

 the skin does not result in subcutaneous or subperios- 

 teal abscess. But experiment has repeatedly shown 

 that such simple injuries, which, if not infected, re- 



