INTRODUCTION. 25 



wounds, which have made possible the wonderful results of 

 modern operative surgery. 1 



In 1834 the parasite of itch (an insect, Acarus scabei) was 

 discovered, and the cause of one contagious malady deter- 

 mined. 



Quite early in the nineteenth century also the relatively 

 large fungi of thrush and some of the parasitic skin diseases 

 were discovered. The bacilli of anthrax, which are also 

 large, were seen in the blood of animals by Pollender in 1855 

 and Davaine in 1863. 



Davaine produced anthrax in animals by injecting into 

 them blood containing anthrax bacilli. But complete proof 

 that these bacilli were the cause of the disease, required that 

 they should produce it when injected alone and when freed 

 from the smallest trace of material derived from the first 

 diseased animal. Unless these conditions were complied 

 with, some other material, for example an enzyme or fer- 

 ment, might be supposed to be carried from the first to the 

 second animal and to be the real cause of the disease. For 

 this purpose it was necessary to cultivate the bacilli in 

 nutrient fluids, such as meat broth, as was done by Pasteur. 

 It then became possible to demonstrate that their properties 

 could remain unaltered after being grown in successive 

 generations on different lots of broth. As bacteria of two 

 or three species were often encountered in mixtures, it be- 

 came most important to secure a method by which the differ- 

 ent species could be separated from one another and be 

 propagated as separate " pure cultures." This was done 

 successfully by diluting such mixtures greatly, so that a 

 drop planted in a new tube of broth should contain only a 

 single organism. The growth ensuing would of course con- 

 sist of the same kind of organism exclusively. Such pro- 

 cedures were uncertain and very laborious. 



1 See History of Medicine, Dr. Roswell Park. 



