62 Causes of Disease. 



products naturally come to reach such a degree of accumulation 

 (concentration) that further tissue change is caused and opportu- 

 nity thus afforded for further penetration by the germs. In this 

 manner rats may be infected by plague by merely dropping upon 

 the unaltered conjunctiva the plague bacteria; and the trypansome 

 of dourine (horse) invades the body by its own motile power 

 through the mucous membrane ; various bacteria, if they can gain 

 entrance into a duct in the nipple of the mammary glands, may 

 multiply luxuriantly in the milk and give origin to intense sup- 

 purative inflammation. 



Microbes gain access to the digestive tract along with food 

 and drink or may be swallowed after being inhaled with dust. 

 The special point of infection (so-called intestinal or food in- 

 fection) in this case is usually the lymph glands of the mouth 

 and pharynx, or of the intestine ; by the same route it is possible 

 that the toxic products of microorganisms, themselves confined to 

 the interior of the intestinal canal, may be absorbed through the 

 blood and lymph channels. 



Microbes gain access to the respiratory tract by inhalation 

 (inhalation infection) with dried dust particles or in the fine dis- 

 charge coughed up by diseased animals and subsequentlv dried. 

 They are in this case in part retained on the nasal mucous mem- 

 brane, in part adhere to the pharyngeal surfaces and may thence 

 pass into the lymph follicles and extend by route of the lymph- 

 atics or be swallowed (giving rise indirectly to an alimentary 

 infection), or they may be carried by the currents of air into the 

 lungs directly. 



Infection of the urogenital mucous membrane usually takes 

 place by transmission of the germs from one to another indi- 

 vidual in the course of sexual congress (coital infection). 



A most favorable opportunity for infection is afforded by 

 wounds of the skin or mucous membrane (wound infection), 

 affording access to the microbes into the lymph spaces and chan- 

 nels of the connective tissue, to the subcutaneous and sub- 

 mucous structures, favoring penetration into the peritoneal cavity 

 and eventually into the blood. The wounds may be so small as 

 to be inappreciable to the unaided eye, as some tiny abrasion of 

 the epithelium ; or it may be that before the disease actually 

 appears there may intervene a considerable period of time, the 

 wound perhaps long healed, so that the point of entrance is en- 

 tirely lost (cryptogenetic infection; kpvwthv, to conceal). This 



