ATTENUATION OF ORGANISMS. 661 



one which is practically uniform in its virulence, must 

 be procured for the attenuation experiments. This is 

 obtained by continued inoculation of the poison on 

 rabbits. If one starts from a rabid dog, and inoculates 

 from this animal a rabbit under the dura mater into the 

 brain substance, rabies appears in the latter animal after 

 an incubation period of about fourteen days ; if the in- 

 oculations are carried on from rabbit to rabbit the period 

 of incubation becomes gradually shorter ; after 40 to 50 

 inoculations it has fallen to seven days, and up till the 

 ninetieth transmission there is scarcely any further 

 alteration to be observed ; hence the poison has become 

 acclimatised in the body of the rabbit, and has attained 

 an almost constant degree of virulence. It has been 

 further proved that the spinal cord of these rabbits con- 

 tains the rabic poison throughout its whole extent. 



If now the cord is cut up into fragments a few ctm. in 

 length, and if these are hung up in dry air, the viru- 

 lence gradually disappears; the length of time which 

 elapses before complete disappearance of the virulence 

 varies with the thickness of the fragments, and above all 

 with the external temperature ; the lower the tempera- 

 ture, the longer is the virulence retained. If the pieces 

 are preserved in a vessel from which air is excluded, or 

 in carbonic acid gas, or in a moist condition, the viru- 

 lence is retained for months, always provided that the 

 access of saprophytic bacteria is prevented. In order to 

 obtain rabic poison of varying virulence suitable for 

 protective inoculations a number of pieces of cord are 

 placed at the same time in a corresponding number of 

 vessels, the air of which is kept dry by pieces of potash. 

 The portions of cord which are kept in this manner for 

 one to two days set up, like the fresh material, rabies in 

 rabbits after seven days' incubation; when the pieces 

 are preserved for six days the length of the incubation 

 period is increased to fourteen days ; when the pieces are 

 kept for seven days in the dry air rabbits inoculated no 

 longer become ill. 



Definite facts as to the active agents are as yet want- 

 ing, nor do we yet possess a sufficient explanation of the 



