10 BACTERIOLOGY. 



The chain of evidence is still more complete if we can from 

 artificial cultures obtain a chemical substance which is capable 

 of producing the disease independently of hving micro-organisms. 



It is of very little value merely to detect or artificially to 

 cultivate a bacterium associated with disease. We must endeavour 

 to estabUsh the exact relationship of the bacteria to disease processes, 

 and the determination of the true pathogenic microbe is beset 

 with fallacies. In many diseases bacteria have been regarded as 

 the actual contagia, until a searching inquiry by other investigators 

 has shown that the evidence was most unsatisfactory or entirely 

 misleading. For example, in diseases with lesions of the external 

 or internal linings of the body, extraneous micro-organisms may 

 get into the circulation and be swept into the internal organs, 

 where they either perish in the battle with the healthy tissues 

 which are opposed to their existence, or they may gain the upper 

 hand, and set up destructive processes. Such organisms, when 

 found in association with these diseases, may be discovered in the 

 blood and internal organs ; and though only accidental epiphytes, 

 often associated with septic comphcation, they may too readily be 

 accepted by the enthusiast as the actual contagium of the disease 

 in , question. 



It is only when such fallacies are exposed that we are brought 

 once more face to face with the fact that the nature of the contagium 

 in hydrophobia, variola, vaccinia, scarlet fever, measles, and many 

 other diseases, is still undetermined. 



