BACTERIA IN DISEASE. l8l 



For example, it is known that the negro race is much less sus- 

 ceptible to yellow fever than the white race. 



Local Conditions. — A most important influence in de- 

 termining the occurrence of infections may be found in local 

 bodily conditions. In endocarditis the lesion usually occurs 

 along the line of closure of the heart-valves, indicating that the 

 point subjected to the greatest friction is the part of the en- 

 docardium most liable to infection. Regions where there is 

 passive hyperemia are more vulnerable, as is seen in hypostatic 

 pneumonia; but on the other hand v. Bier has found that 

 the production of passive hyperemia by artificial means tends 

 to bring about a rapid improvement in local infections. 

 Localities which have suffered from previous inflammation or 

 irritation are rendered more liable to subsequent infection, as 

 when the bladder or pelvis of the kidney containing a calculus 

 becomes the seat of a suppurative cystitis or pyelitis. 



Local conditions become of great importance in surgery. 

 The surgeon can seldom be certain of dealing with a perfectly 

 aseptic wound, and must rely to a large extent upon the power 

 inherent in the fluids and tissues to prevent the development of 

 bacteria. It is important, therefore, to keep the resisting 

 -power of the tissues at the highest possible point. Injury of 

 the tissues disposes the part to infection; so do strangulation and 

 necrosis. In operating, it is to be remembered that hyperemic 

 and edematous parts are more likely to become infected; so 

 also are anemic regions. An infarct of the lung which was 

 originally sterile may be infected with bacteria through in- 

 halation, and undergo suppuration or gangrene. The pres- 

 ence of foreign bodies in the tissues disposes to infection. In- 

 jection of the staphylococcus pyogenes aureus into a rabbit's 

 tissues is not always followed by suppuration, but if a foreign 

 body, like a piece of sterilized potato, be inserted at the same 

 time, infection is much more likely to occur. When lesions are 

 produced in the internal viscera of animals by cauterization or 



