174 MANUAL OF BACTERIOLOGY. 



virus — i. e., from a spontaneous case in a dog — dies in about 

 two weeks, and each succeeding rabbit dies in a shorter and 

 shorter time until ultimately the incubation period is reduced 

 to six or seven days. Beyond this the strength of the virus 

 cannot be increased, and is called virus "fixe," or the fixed 

 virus. Pasteur found, moreover, that the cord of the rabbit 

 which has attained this degree of virulence is attenuated by 

 various agencies, notably by drying, and that animals injected 

 with this attenuated virus can withstand inoculations of more 

 potent virus. By drying for various lengths of time a series 

 of "vaccines" of exactly graded potency is obtained, and 

 starting with the vaccine of least potency an animal can be 

 inoculated with increasingly potent virus until it will with- 

 stand inoculations of the virus fixe itself. 



Omitting all but the chief details, the vaccines against 

 hydrophobia are prepared as follows: 



The cord of a rabbit dead from the subdural inoculation 

 of virus fixe is hung up in a long glass cyhnder in the bottom 

 of which is placed potassium hydrate. The cylinder is placed 

 in a cool place, and every day small bits of the cord are cut 

 off and preserved in a vial of glycerin. The virus which 

 has been dried for thirteen or fourteen days is no longer capable 

 of producing hydrophobia in rabbits, but an animal inoculated 

 with it can withstand inoculation with the cord dried for a 

 shorter time, and after inoculation with the latter withstands 

 inoculations with cord dried for a still shorter time. 



In human beings it is customary to start with the virus 

 which has been dried for nine or ten days, injecting subcutane- 

 ously emulsions of the dried cord, and, if time permits, to 

 give an inoculation every day with virus dried for a shorter and 

 shorter time. As the incubation period for human beings bitten 

 by a mad dog is quite long, — about six or eight weeks, — there 

 is ample time to run in all the inoculations if these are begun 

 promptly, and if in this way the individual is made to with- 



