PATHOGENESIS 13 



into the surrounding medium (extracellular toxins). The 

 diphtheria and tetanus bacilli produce extracellular 

 toxins, and, if a fluid culture be passed through a Pasteur 

 filter, the toxins pass through, giving a toxic filtrate. The 

 bacilli of typhoid, anthrax, plague, and cholera, produce 

 endotoxins, which are retained in the organisms in the 

 filter, and the filtrate has but slight toxic properties. 



Pathogenesis. The application of the adjective ' patho- 

 genic ' to an organism does not signifiy more than that, 

 given a certain set of conditions in a susceptible host, it is 

 able to produce disease. No sharp line of demarcation 

 can be drawn between the pathogenic and non-pathogenic 

 organisms, for even those regarded as harmless may on 

 occasion produce ill-effects, as when the B. subtilis is 

 introduced into the human eye. 



Koctis Postulates. Before an organism can be regarded 

 as specific for a certain disease, it must be shown to con- 

 form to certain requirements which have been formulated 

 by Koch: 



1. The organism must be present in the tissues, fluids, or 

 organs, of the animal affected with, or deadf rom, the disease. 



2. The organism must be isolated and cultivated outside 

 the body on suitable media for successive generations. 



3. The isolated and cultivated organism, on inoculation 

 into a suitable animal, should reproduce the disease. 



4. In the inoculated animal the same organism must be 

 found. 



To these Hewlett adds 



5. Chemical products with a similar physiological action 

 may be obtainable from the artificial cultures of the 

 micro-organism and from the tissues of man or animals 

 dead of the disease. 



6. Specific serum and other reactions, agglutinative, 

 bacteriolytic, complement fixative, etc., are generally 

 obtainable, under certain conditions, if the blood of the 

 infected person or animal be allowed to act on the specific 

 organism producing the infection. 



Although an organism fail to conform to all these con- 

 ditions, other considerations may justify its association 

 with the affection. 



Methods of Spread of Infection. Contagion was the 

 term applied to infective matter when contact with a 

 diseased person was supposed to be necessary for the 



