SUSCEPTIBILITY AND RESISTANCE, 143 



injuring or diminishing the vitality of a part. If, for 

 example, previously to an intravenous injection of staphy- 

 lococci, the aortic cusps of a rabbit be injured, the 

 organisms may settle there and set up an ulcerative 

 endocarditis ; or if a bone be injured, they may produce 

 suppuration at the part, whereas in ordinary circumstances 

 these lesions would not take place. The action of one 

 species of bacterium is also often aided by the simultaneous 

 presence of other species. In this case the latter may act 

 simply as additional irritants which lessen the vitality of the 

 tissues, but in some cases their presence also favours the 

 development of a higher degree of virulence of the former. 

 These facts, established by experiment (and many others 

 might be given), illustrate the important part which local or 

 general conditions of diminished vitality may play in the 

 production of disease in the human subject. This has long 

 been known by clinical observation. In normal conditions 

 the blood and tissues of the body, with the exception of 

 the skin and certain of the mucous surfaces, are bacterium- 

 free, and if a few organisms gain entrance, they are de- 

 stroyed. But if the vitality becomes lowered their entrance 

 becomes easier and the possibility of their multiplying and 

 producing disease greatly increased. In this way the 

 favouring part played by fatigue, cold, etc., in the produc- 

 tion of diseases of which the direct cause is a bacterium, 

 may be understood. It is important to keep in view in 

 this connection that many of the inflammation-producing 

 and pyogenic organisms are normally present on the skin 

 and various mucous surfaces. The action of a certain 

 organism may devitalise the tissues to such an extent as to 

 pave the way for the entrance of other bacteria ; we may 

 mention the liability of the occurrence of pneumonia, 

 erysipelas, and various suppurative conditions in the course 

 of or following infective fevers. In some cases the specific 

 organism may produce lesions through which the other 

 organisms gain entrance, e.g., in typhoid, diphtheria, etc. 

 It is not uncommon to find in the bodies of those who 

 have died from chronic wasting disease, collections of micro- 



