58 Proceedings of the Royal Irish Academt/. 



later all the imvaccinated sheep were dead — the others were in perfect 

 health. 



This work formed the foundation of the modern vaccine treatment of 

 disease, whereby a defence-mechanism is built up in the subject through 

 repeated injections of a weakened form of the disease. The process of 

 weakening the virulence of a disease germ by chemical or other methods is 

 still termed Pasteurisation. 



TI. 



The winter of his life found Pasteur engaged in the development of 

 bacteriology and preventive medicine. His last scientific conflict was with 

 an invisible foe — the ultra-microscopic organism which gives rise to rabies 

 (or hydrophobia). Up to the present time the actual cause of this fearful 

 disease is absolutely unknown. A large number of organisms have at one 

 time or a.nother been isolated as the excitants of the disease, but none has 

 stood the tests of extensive investigation. The virus of the disease can 

 pass through filters capable of retaining known germs, and hence the infective 

 organism must be capable of existing in a very minute form at one period of 

 its life-history. 



The infective agent, whatever its nature, is found in the saliva of animals 

 stricken with the disease, and hence is most frequently conveyed by bites of 

 rabid animals. It is also found concentrated in the brain and spinal cord. 



Pasteur first showed that when an extract made from the brain-matter of 

 a mad dog was injected into the nervous system of a healthy dog, the dog 

 developed rabies after an incubation period of fourteen days. 



If the healthy dog were inoculated with the virus by means of a wound 

 on the surface of its body, a much longer and more variable period intervened 

 before the disease appeared. 



Pasteur then discovered that by drying the spinal cords taken from rabid 

 animals for varying lengths of time he could prepare a series of viruses of 

 gradually decreasing strengths. If a spinal cord were dried for fourteen days 

 or longer, it lost all its toxic properties. 



By these means he was able to obtain a series of vaccines or preparations 

 of attenuated virus just as he had succeeded in the case of anthrax. 



By injecting small quantities of this attenuated virus in gradually 

 increasing strengths each day, Pasteur was able to build up a defence 

 mechanism which rendered an animal completely immune from rabies. 

 Owing to the long incubation period of the disease, at least fourteen days 

 from infection, it was possible to construct this wonderful defence while the 

 organisms of the disease were actually in the animal's body. Hence, Pasteur's 



