528 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



inhaled into the nose or throat. Some of these diseases may some- 

 times be transmitted by intermediate agents, clothing, etc. (fomites). 

 In the last three diseases, which are known as the venereal diseases, 

 an abrasion of the integument is a prerequisite and the infectious agent 

 must enter therein. This is usually brought about by absolute contact 

 of one individual with another. In leprosy also almost direct contact 

 is necessary for a transfer of the infectious agent. 



THE ROUTES BY WHICH INFECTIOUS MICROORGANISMS 

 ENTER THE BODY. 



Microorganisms enter the body through either the external or in- 

 ternal surfaces. It has been shown that healthy and intact skin 

 furnishes an efficient barrier to the entrance of infectious agents. Path- 

 ogenic bacteria, for example the streptococcus and the various varieties 

 of the staphylococci, are present on the skin almost continuously, yet do 

 not often produce infections. When there is an abrasion of the skin or a 

 diseased duct or hair follicle the bacteria frequently pass down and an 

 infection results. Then again, when the above-mentioned pyogenic 

 microorganisms are vigorously rubbed into the skin, infection sometimes 

 takes place, but in this instance there has been some mechanical injury 

 to the skin. Minute and unobserved abrasions of the skin also serve as 

 points of entrance for the Bact. pestis of the plague. The microorganisms 

 of tetanus, anthrax, symptomatic anthrax and malignant oedema may 

 enter the skin through wounds. Sometimes the infectious agents remain 

 local and at other times are carried from the point of the original 

 entrance and this may take place in different lengths of time. For 

 example, in tetanus the bacteria remain localized at the point of the 

 original wound and disseminate their toxin from this point. In the 

 various pyogenic infections the bacteria usually remain localized. How- 

 ever, in anthrax the bacteria are carried into the circulation in a very 

 few minutes after they enter the wound. In the new-born, infection 

 very frequently enters the body through the umbilicus or navel. Tetanus 

 is one of the common diseases acquired in this way. Microorganisms 

 may also enter the skin through the wounds made by insects such as 

 mosquitoes, ticks and fleas. The clearer cut the wound the less the 

 danger of infection because of the mechanical and bactericidal barrier 

 and action of the fibrin and blood serum. A free flow of blood also 



