DISEASES PROIHTKD BY UI/TRA-MICROSCOPIC ORGANISMS 491 



method of vaccination at intervals with attenuated virus. The 

 virus is found, on experimentation, to be rather variable in its 

 power to produce disease. The virulence is exalted by repeated 

 inoculations of rabbits until it becomes the " fixed " virus of 

 Pasteur, and will kill rabbits in six to seven days. This is then 

 injected into a rabbit, and upon its death the spinal cord is care- 

 fully removed with all aseptic precautions, and suspended in a 

 desiccator over caustic potash. It is kept at a constant tempera- 

 ture of 23 in the absence of light for two weeks. The vaccine 

 consists of an emulsion of this cord in physiological salt solution. 



Fig. 213. Removal of the spinal cord from a rabbit (Stimson, Bull. Xo. 65, 

 Hygienic Laboratory). 



Later, injections are made with a cord that has been dried for a 

 shorter period. Repeated injections are made. The fact that the 

 disease has normally a long incubation period gives an oppor- 

 tunity in the human for the use of this method. The active 

 immunity established by the injection of the attenuated virus is 

 sufficient to destroy the infecting organism. This method of 

 treatment has been highly successful when commenced in time. 

 It is still strictly an active immunity. To what principle it is due 

 is not known. 



Bacteriological Diagnosis. The disease may be diagnosed by 



