206 GENERAL BIOLOGY OF MICRO-ORGANISMS 



buncles; but to most microbes the uninjured skin presents an 

 elective barrier. 



The question whether infectious agents may penetrate epithe- 

 lium and gain the lymph or blood-vessels beyond without causing 

 a local lesion, has received considerable attention and it seems 

 to, be established as certainly possible in the intestine during 

 the absorption of fat, and it may perhaps occur in other locations. 



Infection through wounds, even minute breaks in the epithe- 

 lial covering, is very common. Such wounds made, by insects 

 are the common portals of entry for the germs of malaria, plague, 

 yellow fever, relapsing fever and many more diseases. Larger 

 wounds nearly always become infected with pyogenic cocci 

 unless they are properly cared for. The introduction of infectious 

 material into the subcutaneous tissue may occur accidentally in 

 deep wounds and is a common mode of inoculation in the labora- 

 tory. Infection with the anaerobic bacillus of tetanus frequently 

 occurs in this type of wound. 



Infections of the peritoneal cavity, pleural cavities and cavi- 

 ties of the joints result from penetrating wounds, by the entrance 

 of bacteria from contiguous tissues, as through the intestinal 

 wall into the peritoneal cavity, and through the blood and lymph 

 channels. 



Local Susceptibility. — The invading parasite is favored, by 

 conditions of local susceptibility such as tissue destruction, 

 presence of necrotic tissue and foreign bodies, and also by the 

 presence of other infectious- microbes. Small-pox and staphylo- 

 coccus, tetanus and the pus cocci, scarlet fever and streptococcus, 

 are common examples of such mixed infections. In some in- 

 stances one infection predisposes to another. For example, 

 measles is likely to favor "the development of tuberculosis; the 

 caseous tubercle is likely to be invaded by the streptococcus. 

 These subsequent invasions are spoken of as secondary infections. 



Local and General Infections. — The invading microbes may 

 remain localized near the point of entrance, as for example in 

 tetanus and diphtheria. In such cases the general effects may be 



