CHAPTER VII. 



PORTALS OF ENTRY OF INFECTIOUS AGENTS 

 AND OF PARASITES INTO THE BODY. 



Microbes must observe a more regular manner of 

 entering the body than leaving it, if they would produce 

 disease. This is due to the fact that tissues differ in 

 vulnerability to their attack, being favorable to the 

 growth of some germs, and unfavorable to others. The 

 same variation in susceptibility is noticed in the various 

 organs of the body, and in different localities even 

 where the tissues are not perceptibly dissimilar. From 

 this is would appear that bacteria are preferential in 

 their action, a conception that is quite generally believed. 

 However, while admitting that infectious agents have a 

 selective action, we must not overlook the part taken by 

 the body in limiting microbic action through its secre- 

 tions and anatomical peculiarities. Thus it is well 

 established that the liver, through its secretions, has 

 astonishing germicidal powers, and that to this influence 

 we undoubtedly owe immunity from many infections. 

 Again, the comparative freedom of the eye from infec- 

 tion is largely a result of winking, by which act mechan- 

 ical removal of bacteria is accomplished. As a conse- 



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