22 VETERINARY BACTERIOLOGY 



theories were not generally accepted, and it was not until 1840 

 that Henle proposed what we have come to call the germ theory 

 of disease. He never succeeded in proving his point satisfactorily 

 because of the lack of proper methods and technic. Many 

 other writers within the next few years discussed the theory 

 and numerous facts were adduced in favor of it. The majority 

 of medical practitioners, however, put very little faith in it. The 

 argument that certain organisms were always present was met 

 with the statement that these organisms were the result and not 

 the cause of the disease. Davaine (1863) practically demon- 

 strated by inoculation experiments the causal relationship of a 

 bacillus he found in the blood of diseased animals to anthrax. 

 Pasteur (1865) proved the cause of a silkworm disease to be a 

 protozoan parasite. Koch and Pasteur later cultivated the 

 anthrax organism in the laboratory and showed beyond a doubt 

 its relationship to the specific disease. Improved laboratory 

 technic cleared up the cause of many diseases within the next de- 

 cade or two. The discovery of the Bacillus tuberculosis by Koch 

 (1882) marks the real beginning of bacteriologic science. The 

 knowledge of protozoa as a cause of disease lagged somewhat 

 behind that of bacterial infections. Evans (1880) described 

 the trypanosome of surra and transmitted the disease by inocula- 

 tion experiments. In 1882 Laveran observed the Plasmodium 

 malarice, the cause of malaria. 



Development of Laboratory Methods. Progress was delayed 

 in the study of objects as minute as the bacteria because of the 

 lack of proper methods for their isolation, observation, and identi- 

 fication. Culture-media in which the pathogenic microorganisms 

 could be grown were used by Pasteur and Koch. To the latter 

 we are indebted (1882) for our knowledge of the solid media which 

 can be used for the isolation of organisms from mixed cultures. 

 The importance of this contribution can hardly be overestimated, 

 for the use of pure cultures lies at the very foundation of all 

 modern bacteriologic investigation. This one discovery accounts 

 in large measure for the rapid advance made during the next 

 two decades in the identification of the organisms producing 

 disease. The use of aniline dyes in rendering cells and their struc- 

 tun more plainly visible under the microscope we owe to Weigert, 



