PHENOMENA OF INFECTION. 69 



marily enter the body, yet a wealth of facts gleaned 

 through experiments upon animals leads us to conclude 

 their numbers must be large. Clinical observations 

 also furnish some data in support of this view. In 

 cases where long and tedious operations are performed, 

 a wound is often exposed to the air for hours without ill 

 effects following. Certainly no one believes that 

 during the time of exposure and manipulation bacteria 

 do not soil the wound. On the contrary, we know they 

 do; but the reason harm only occasionally results from 

 those that enter is attributed to the fact that either they 

 are present in insufficient numbers and are of low 

 virulence, or the vital forces of the patient are sufficient 

 to destroy them. Similar observations are made by 

 physicians in the repair of injuries in which either 

 aseptic or antiseptic precautions are utterly impossible. 

 Thus in the practice of mid-wifery, for example, al- 

 though such unfortunate surroundings are quite com- 

 mon, cases of puerperal fever (child-bed fever) are 

 so infrequent as to excite comment. Indeed, puerperal 

 fever occurs occasionally under conditions in which 

 the surroundings are ideal, probably through the ful- 

 fillment of the third condition above given, viz., extra- 

 ordinary virulence of the germs. 



A singular peculiarity of pathogenic micro- 

 PoRTAL OF organisms is that in order to provoke dis- 

 Entry. ease, they must find lodgment in or upon 



that portion of the body which offers the 



