112 MICROBES AND TOXINS 



"witnesses" of the diseases, and simply the inhabitants of 

 lesions which they had not produced — ^just as the same moulds 

 are usually found in pots of the same jam left open to the air. 



The specificity of infectious diseases has been demonstrated 

 by Pasteur and Koch. 



Since there is no " spontaneous generation," at least in the 

 world of the present day, it is impossible for the micro- 

 organisms to originate in the diseased tissues, and the science 

 of fermentation has proved that a given cell, inoculated in a 

 sterile fluid of known composition, produces in it definite and 

 constant phenomena. These ideas were taken up by medicine 

 when Davaine maintained that the bacteridium was the 

 cause of anthrax, and when Obermeier held that recurrent 

 fever was caused by the spirochsete found in the blood of the 

 patients during the fever. 



To prove the specific activity of a ferment it is necessary to 

 isolate and make pure cultures of it and to re-inoculate it. It 

 was this that we learnt from Pasteur and Koch. Koch's 

 memorandum on tuberculosis remains the complete model 

 for the discovery of the microbe of a disease and of the 

 demonstration of its specificity. 



Specificity of function is the point of capital importance. 

 The fixity of form in bacteria is of great use in the search for 

 and identification of them ; but on this latter point science has 

 been obliged to become less exacting ; the form of the bacteria is 

 not always a sufficient distinguishing character. They are 

 really defined by their chemical and physiological actions. 

 Thus the tubercle bacillus is better defined by its staining 

 peculiarities (a physico-chemical reaction) than by its shape ; 

 still better by the appearance of pure cultures than by staining; 

 and better still than by the cultures, by its excretion of 

 tuberculin. Finally, the study of the reactions of the body, 

 i.e., of immunity, has shown that the cells of the patient respond 

 in a specific manner to the attack of the bacterial cells, and both 

 medicine and hygiene daily employ the property of specificity 

 in the antibodies. 



The " Para " and " Pseudo " Bacteria.— We may now 



