BACTERIA IN DISEASE. 167 



tain. Welch says, " the fact that some individuals are 

 attacked, and others, apparently equally exposed to the 

 danger of infection, escape, is not always due to any espe- 

 cial predisposition on the part of the former. It may be 

 that the germs hit the one and miss the other, and we 

 would have no more right to say that the former are espe- 

 cially predisposed than to say that those who fall in battle 

 are predisposed to bullets and those who escape are bullet- 

 proof." It is probable that the importance of an hereditary 

 tendency to certain infections, notably tuberculosis, has been 

 overrated. 



Race. The influence of racial predisposition is undeni- 

 able. For example, it is known that the negro race is much 

 less susceptible to yellow fever than the white race. 



Local conditions often have a most important influence 

 in determining the occurrence of infections. In endocar- 

 ditis 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 endocardium most liable 

 to infection. Regions where there is passive hyperemia 

 are more vulnerable, as is seen in hypostatic pneumonia. 

 Localities which have suffered from previous inflammation 

 or irritation are rendered more liable to subsequent infec- 

 tion, as when the bladder or pelvis of the kidney contain- 

 ing 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 per- 

 fectly 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 



