INFECTION AND IMMUNITY. 543 



been caused by a poison elaborated by or escaping from 

 the locally deposited organisms and carried to distant 

 parts of the body by the circulating fluids. 



In tuberculosis the nodules resulting from inoculation 

 with the dead bacteria must have been the result of a 

 poison associated with the bodies of those dead bacteria 

 and liberated on their disintegration in the tissues. 



While in diphtheria it is plain that its characteristic 

 manifestations are the outcome of a poison produced 

 locally by the growing bacteria and carried thence by 

 the circulating fluids to distant organs, there to exhibit 

 their destructive properties. 



Thus far, then, infection must be viewed as a conflict 

 between bacteria on the one hand and tissues on the 

 other ; the former having as their weapons of offence 

 destructive poisons ; the latter, vital defensive provisions 

 that enable them to resist infection with greater or less 

 degree of success, according to circumstances. It makes 

 no difference, therefore, whether, in infection, the bacteria 

 be generally or only locally present, the mechanism is at 

 bottom a destructive intoxication. 



BACTERIAL TOXINS. Through special investigations 

 that have been made upon the products of growth of 

 many pathogenic bacteria the opinion that infection is 

 fundamentally a chemical process receives further con- 

 firmation. It has been found possible by the use of ap- 

 propriate methods to isolate from among the mass of 

 material in which certain of these organisms have been 

 artificially cultivated substances which, when separated 

 from the bacteria by which they were produced, possess 

 the power of causing in animals all the constitutional 

 symptoms and pathological tissue-changes that occur in 



