96 PRINCIPLES OF VETERINARY SURGERY 



fectly innocuous in others. Some will kill the weak and 

 prove harmless to the strong, etc. However, any bacterium 

 that is capable of producing derangement of the living cells 

 deserves to be classed among the pathogenic micro-organ- 

 isms, whether the lesion produced is a simple local phenom- 

 enon in a particular species, or a grave or fatal malady in an- 

 other. Besides the variable disease-producing quality seen 

 in different species, and under different conditions in the 

 same species, no two given samples of the same micro-organ- 

 ism possess the same pathogenic capacity. The virulence of 

 microbial! cultures is variable. No two inoculations, experi- 

 mental or accidental, will produce precisely the same degree 

 of disease. Various influences are capable of attenuating 

 micro-organisms to the point of enfeebling and even de- 

 stroying their pathogenic powers. Unnatural environment, 

 heat, cold, desiccation, prolonged exposure, their number, 

 the point of entrance, etc., are among the conditions which 

 modify their virulence. Furthermore, the pathogenicity of 

 many micro-organisms is materially increased by inoculating 

 them with other bacteria-favoring bacteria. The tetanus 

 bacillus is harmless unless associated with other microbes; 

 the bacillus of malignant oedema may be increased in viru- 

 lence by inoculating it with the bacillus prodigiosus, etc. 



The disease-producing power of bacteria depends upon 

 various properties which differ with the different forms. The 

 pathogenicity, however, largely depends upon their ability 

 to elaborate poisonous products — toxins, toxalbumins or 

 bacterio-proteins, which are absorbed in sufificient quantities 

 to intoxicate the organismi. "The intoxication is, however, 

 but a part of infection which consists of the sum of all the 

 vital phenomena manifested by the bacterium in its parasitic 

 life." (McFarland.) The toxins are not all thoroughly un- 

 derstood. They are unstable substances, with few excep- 

 tions, that are readily destroyed by exposure to light, air. 



