80 PATHOGENIC BACTERIA. 



bugs, fleas, etc., are allowed to prey upon infected mice 

 and then upon healthy mice the latter did not become 

 infected even when the biting insects were so pressed 

 upon as to play the role of injecting syringes. The ba- 

 cilli of the diseases mentioned occurred in vast numbers 

 as long as ninety-six hours in the excrement of the in- 

 sects, then disappeared, losing their virulence and vitality 

 before this period had passed. 



b. The Mucous Membranes. — In the case of the moist, 

 soft mucous membranes it does not seem necessary that 

 a breach of continuity should occur for infection to take 

 place. The lodgement of bacteria upon the surface, their 

 multiplication, the effect of their toxin in producing super- 

 ficial necrosis, probably precedes in many cases actual 

 entrance of the bacteria into the tissues. Such a mode of 

 operation suggests itself especially in such affections as 

 diphtheria and gonorrhea, where exposure is sufficient 

 guarantee of infection without a necessary injury. 

 Doubtless in many cases of mucous membrane infections 

 the entrance of the bacteria into the tissues is aided by 

 phagocytes, especially where the relation of bacteria to 

 the cells is intimate, as in gonorrhea. 



2. The Respiratory Apparatus. — We are unable to say 

 in how many of the specific infectious diseases the infec- 

 tion takes place through the inspired air ; but it is in all 

 probability frequent. Diseases such as small-pox, scarla- 

 tina, measles, etc., in which infection occurs without 

 actual contact with the patient, but simply from being 

 near him, seem to depend upon the inspiration of the 

 contagium in the air. Pneumonia, tuberculosis, influenza, 

 and a number of diseases whose causes are known to us 

 are also inhalation infections. 



It is probable that little difficulty is experienced by the 

 pathogenic bacteria in penetrating the alveolar mucosa, 

 which seems to be destructive to the non-pathogenic 

 forms. Buchner's experiments upon the inhalation of 

 anthrax spores are of interest as illustrating the co-opera- 

 tion of factors in infection. He found that when rabbits 



