212 BACTERIA IN DAILY LIFE 



it has been found that very much smaller quantities 

 produce a fatal result than when subcutaneously 

 inoculated. Thus, a rabbit which received two 

 cubic centimetres of the poison under the skin 

 took four days to succumb to tetanus, whilst one- 

 twentieth of the quantity inoculated into the brain 

 sufficed to kill another rabbit of the same size in 

 less than twenty hours. 



Another very instructive example of this sus- 

 ceptibility of the nerve-centres for certain poisons 

 is afforded in the case of rats and the toxin of 

 diphtheria. Rats possess a natural immunity from 

 this substance, and can successfully withstand a 

 dose of diphtheria poison introduced under the 

 skin which would infallibly kill several rabbits. 

 This state of immunity, however, entirely dis- 

 appears when the toxin is brought directly in 

 contact with nervous tissue, for a very small 

 quantity of diphtheria poison insufficient to 

 cause under ordinary circumstances even a pass- 

 ing swelling at the seat of inoculation will, 

 when introduced into the brain of a rat, kill the 

 animal. 



Again, rabbits are generally credited with 

 possessing high powers of resisting the action of 

 morphia, a large dose of this substance introduced 

 subcutaneously producing no result whatever. A 

 cerebral inoculation, however, of a minute quantity 



