EXPERIMENTAL IMMUNITY. 119 



ized cultures or toxins ; by inoculation with cultures of the 

 specific germ mixed with other organisms; with dead bacteria; 

 with germs grown on media containing an antiseptic. Very 

 virulent organisms are attenuated by growing them at high 

 temperatures. 



Immunity secured in this way is of only short duration, 

 but is sufficient to protect the individual against the disease 

 for the time being. 



The toxicity of the virus may also be modified by heat- 

 ing it; or by the addition of chemicals, such as iodine or 

 chlorine. 



A certain amount of tolerance (really a form of immunity) 

 may be produced by substances other than bacteria. This is 

 seen in many forms of intoxication. The fact that immunity 

 may be conferred not only by pathogenic bacteria and their 

 products, but also by dead bacteria, saprophytes, and inert 

 particles, forces us to the conclusion that whatever confers 

 the immunity is a chemical compound which is distributed 

 widely in nature. Calmette produced immunity by successive 

 injections of snake-venom. Ehrlich's experiments with ricin 

 and abrin were similarly successful. Others produced immu- 

 nity to poisonous eels, botulismus, arsenic, etc. The immu- 

 nity produced by these substances is practically the same as 

 bacterial immunity. The immunity is due perhaps to phago- 

 cytosis or phagolysis, and the tissue becomes enriched with 

 antibacterial substances. 



Passive form : This form of immunity is always experi- 

 mental. It is produced by the administration of antitoxins. 

 An antitoxin is a substance derived from immunized animals, 

 and which, when injected into another animal, confers immu- 

 nity. A special chapter will be devoted to the consideration 

 of these highly interesting substances. 



Tissue-suspensions are also a source of experimental 

 immunity. Wassermann found that the spinal cord of a 

 rabbit when crushed and suspended in a normal salt solution 

 and mixed with tetanus toxin protected against tetanus even 

 when the cord alone w r as injected either before or after the 

 toxin, or into another part of the body. These findings have 

 been confirmed by others; but it is maintained that the 



