320 BACTERIA, 



dura mater (the outer skin or covering of the brain) produced 

 similar effects. Nerves were also found to contain the virus, 

 but the saliva, as we have seen, and the salivary glands 

 introduced in the above manner produced a much more 

 virulent and rapid form of the disease than the other tissues 

 and fluids mentioned : quite as virulent a form, in fact, as 

 when the animal was actually bitten by a rabid animal. 



Finding that these tissues and fluids taken from a rabid 

 animal varied in their virulence, and knowing that in the 

 case of anthrax virus the virulence may be diminished or 

 increased by inoculating into an animal of another species, 

 he made another series of experiments, as a result of which 

 he found, that although virus taken from similar positions, 

 say, the cerebro-spinal fluid, had always the same action in 

 the same species, when the fluid was taken from an animal 

 of a different species it was weaker or stronger as the case 

 might be. Thus in a dog the virus is of constant strength, 

 and inoculations made from dog to dog kill the animal with 

 the same incubation period, the same symptoms, and practi- 

 cally in the same time. When inoculated from the dog to 

 the monkey, however, the virus becomes less virulent ; it 

 is said to be attenuated or weakened, the attenuation be- 

 coming more and more marked in successive inoculations 

 from monkey to monkey ; the course of the disease becomes 

 longer and longer, until eventually there may come a time 

 at which the virus, when introduced under the skin or into 

 the cranial cavity, is not sufficiently active to cause the death 

 of this species. If this attenuated fluid be now inoculated 

 into a rabbit, a dog, or a guinea-pig, it still remains com- 

 paratively weak for a time, through successive inoculations 

 on these animals z'.e. y at first it does not kill, then it kills, 

 but only after a considerable time ; but gradually the 

 virulence returns, until at length it reaches its original level 

 of malignancy ; whilst, if the successive inoculations are 

 made in rabbits with primary fluid from either the dog or 

 the monkey, the virulence may become so exalted that it is 

 considerably greater even than that of the virus taken from 

 the street dog, which at one time was supposed to be the 

 most virulent form except that of hydrophobic wolves, which 

 has always been known to be specially fatal ; the virulence 

 is doubled as the inoculation period is reduced to about one 

 half. 



