THEORIES OF IMMUNITY. 7 



the microbes of plague and cholera, Pasteur and 

 his co-workers turned to the study of hydropho- 

 bia. All efforts to cultivate the virus from the 

 spinal cord of rabid dogs failed, although inocula- 

 tion experiments proved its presence in this struc- 

 ture. The unique idea then occurred to consider 

 the infected spinal cord as a fully developed cul- 

 ture of the virus. It remained to subject such a 

 culture to the proper attenuating conditions for 

 the purpose of weakening or actually destroying its 

 virulence in order to make it fit for protective in- 

 jections. This was accomplished by drying the 

 cords in a closed vessel over a hygroscopic sub- 

 stance (solid potassium hydroxid), the final viru- 

 lence of the cord depending on the length of time 

 it had been subjected to the drying process. The 

 technic of the protective injections, the success of 

 which is household knowledge, will be a subject for 

 later consideration. 



Of primary importance, during this period, was T wo 

 the work of Koch on the specific bacteria of tu- 

 berculosis, cholera, typhoid and the pyogenic dis- 

 eases; and not least his improved methods of ob- 

 taining pure cultures through the use of solid 

 media (gelatin) on plates. Through his work and 

 that of Pasteur two great principles had been 

 set in motion ; the microbic specificity of infectious 

 diseases, and protective inoculation in its general- 

 ized form, through the use of attenuated virus. 



The scientific mind turned at once to the in- Theories of 



TTTI i *iii * ne Cause 



quiry, What changes in an animal body are re- O f immunity. 

 sponsible for the immunity which is acquired as 

 the result of protective inoculations? Also, upon 

 what properties of the tissues or body fluids does 

 the natural immunity of an animal depend, and 



