130 ESSAYS ON BACTEEIOtOGY. 



formation. They settle tlie questions of duration atnl 

 degree of virulence in germs of varying activity. 

 They are the crucial tests of specific preventive and 

 curative methods and agents, such as the antitoxin and 

 antitoxic treatment of diphtheria. Finally, they are 

 used to determine, with each infectious disease, the 

 various possible channels of infection. 



Bacteria are inoculated on the skin, into and under 

 the skin, into the solid tissues and the body cavities. 

 They are introduced by feeding, by inhalation, and 

 even directly into the blood current. Thus, if the 

 supposed germs of erysipelas be under investigation, 

 they would be tested upon superficial wounds. Those 

 of cholera and typhoid fever would naturally be tried 

 by feeding; those of tuberculosis and suppuration by 

 all the methods. If we suspect that general miliary 

 tuberculosis is the result of an irruption of tubercular 

 material into a blood vessel, we may test the suspicion 

 by inoculations of tubercular cultures into the blood. 

 If we suspect the importance of a combination of non- 

 bacterial and bacterial factors in the prodiiction of a 

 disease, we may settle the question by experiments 

 with the former agents, then with the bacteria, and 

 then with both together. Such considerations reveal 

 the significance and value of inoculation experiments 

 in bacteriology, and their necessity in promoting its 

 beneficent work. 



The history of bacteriology in its relation to prac- 

 tical medicine is similar to that of many other con- 



