146 BACTERIOLOGY FOR NURSES 



Other investigators sought to obtain the desired result by heating 

 the blood of animals suffering from the disease for a few minutes 

 at 55 C. or by heating cultures of the organism at 80 C. and 

 then using them for inoculation. Neither of the methods were 

 very successful. After much experimentation Pasteur found that 

 by cultivating the organisms at a high temperature, 42 to 43 C., 

 they could be so attenuated that finally they were entirely robbed 

 of their disease-producing power. In this manner they could be 

 modified at will, and by inoculating animals first with a highly 

 attenuated culture and then with a moderately attenuated one 

 he was able to immunize them against anthrax. 



Immunization against hydrophobia was the next study under- 

 taken by Pasteur, and here as in the two previous cases his efforts 

 were successful. Again he was confronted with a difficulty not 

 met with before ; in this case the causal agent was unknown. 

 First, Pasteur established the fact that the virus of rabies finds 

 lodgment in the brain and spinal cord since by injecting an emul- 

 sion of these tissues taken from an infected animal into rabbits 

 he was able to reproduce the disease. Then he discovered that 

 if the spinal cords were removed from these rabbits and subjected 

 to a drying process the virulence of the virus contained in them 

 could be attenuated to whatever degree wished, depending upon 

 the length of the period of drying. 



Pasteur taught, then, at least three methods of so modi- 

 fying organisms that they may be used for the artificial pro- 

 duction of active immunity : (1) prolonged cultivation on media, 

 (2) growing at a high temperature, and (3) drying. He also 

 demonstrated at the same time that the causal agents of each 

 disease have their own characteristics and must be dealt with 

 accordingly. 



In certain diseases which are greatly influenced by the channel 

 of entrance of the invading organisms still another method is 

 frequently employed ; the most resistant tissues are chosen as the 

 site of inoculation. In the case of cholera, for example, there is 

 much less danger in injecting living organisms into the subcutane- 

 ous tissues than in taking them by mouth. 



