POISONS AND THEIR PREVENTION 171 



as it has inaugurated a new departure in methods 

 of immunisation. 



The previous methods in vogue for inducing 

 immunity in animals from a particular disease 

 consisted in converting the virus itself into a 

 vaccine, as was done by Pasteur in his classical 

 investigations on anthrax and its prevention ; and 

 secondly, the employment of anti-toxic serums, in 

 which the virus is not directly inoculated into the 

 animal to be protected, but in which an inter- 

 mediary is employed between the virus and its 

 victim. This intermediary, or living machine 

 for the generation of the anti-toxin, is usually 

 a horse, which is artificially trained by being 

 given gradually increasing doses of the virus or 

 toxin, until it ultimately withstands doses which 

 in the first instance would infallibly have killed 

 it. When the animal has arrived at this satis- 

 factory stage or condition of complete immunity, 

 some of its blood is from time to time drawn 

 ofif, and the serum thus obtained constitutes 

 the anti-toxin which now figures so prominently 

 in modern therapeutics. Besides diphtheria-anti- 

 toxic serum there are also those of tetanus, or 

 lock-jaw, plague, the famous anti-venene serum, 

 about the discovery and preparation of which 

 greater detail is given later on, and many others 

 which are still the subject of experimental inquiry. 



