INFECTION AND INTOXICATION. 79 



this relation, for in ordinary injuries probably very few 

 bacteria are admitted to the tissues. 



Infection from external sources, while much less fre- 

 quent than the autochthonous forms, are commonly much 

 more severe, especially when the infecting bacterium has 

 been operating in another animal of the same species. 

 This increased severity probably depends upon increased 

 virulence of the bacterium attained during its residence 

 in the tissues. 



The infecting bacteria come to us through all conceiva- 

 ble fomites. The dust in the air, the articles of food 

 and drink, the soil we walk upon, the bodies and dis- 

 charges of diseased animals, all contain infective micro- 

 organisms. 



The avenues of infection are numerous. 



1. The Surfaces of the Body. — These are all covered 

 with epithelial protections which vary in character and 

 thickness according to necessity. 



a. The Skin. — It is very improbable that bacteria, 

 either resident upon the skin or accidentally brought in 

 contact with it, can penetrate its uninjured structures. 

 Only microscopic injuries are necessary, however, and 

 infection can be produced by gently rubbing the bacteria 

 into tissue, which is slightly abraded in the process. 



Any wound, a puncture, incision, laceration, abrasion, 

 etc., that destroys the perfect continuity of the epithelium 

 is a point of entrance for bacteria upon the skin, upon 

 the instrument, or in the air. 



The bites of animals and insects may prove dangerous 

 — as in rabies of dogs, and tse-tse fly-bites of herbivora — 

 from bacteria in the saliva. There has been a tendency 

 to pay a great deal of attention to the bites of insects since 

 the study of plague has occupied us, and for a time it was 

 supposed that the infection might readily be produced 

 by the bites of parasites. Nuttall 1 has, however, fully 

 investigated the subject in relation to anthrax, chicken 

 cholera, and mouse septicemia, and found that when bed- 



1 Centralbl.f. Bakt. u. Parasitenk., April 12, 1898, xxiii., No. 14. 



