ACQUIRED IMMUNITY 121 



197. But let us go a little deeper. Pasteur killed rabbits 

 suffering from rabies and dried their spinal cords. His 

 treatment for the cure of an infected person began by the 

 injection into him of an emulsion prepared from a cord which 

 had been dried for fourteen or fifteen days. This cord was 

 quite incapable of causing disease ; presumably the patho- 

 genetic organisms had perished. The treatment was con- 

 tinued by injections from fresher and fresher cords, and 

 ended by the injection of material from a cord that was abso- 

 lutely fresh and intensely virulent. By this method he 

 enabled many individuals who had been infected and who 

 would otherwise have perished to acquire immunity. It is 

 clear, since the antitoxin is procurable from old and dried 

 cords, not from those which are absolutely fresh and virulent, 

 that the immunizing substance is here produced in the dead 

 and drying cords in the presence of dead or dying micro- 

 organisms, not in the living and virulently infected animals 

 from which the cords are taken. Moreover, did the living 

 infected animal produce the antitoxin it would survive, 

 whereas it invariably perishes. In this case then the anti- 

 toxin is not produced by the vital action of the cells of the 

 rabbit, but arises in the absence of their vital action in a 

 dead thing. It is wildly improbable that the pathogenetic 

 organisms produce toxins when in health, and substances 

 which chemically neutralize the toxins when dying or dead. 

 No doubt chemical changes do occur in a cord, but it would 

 be equally strange if these resulted in the formation of sub- 

 stances antagonistic to this particular toxin and no other. 



198. Again, the parasites of some diseases are capable of 

 infecting more than one species of animal. Under the changed 

 conditions they generally, if not always, vary the virulence 

 of their toxins. Thus rabies in dogs is a very deadly disease. 

 If passed through a series of rabbits its severity is still more 

 exalted, and it is even more quickly fatal. If passed through 

 a series of monkeys it becomes so mild as to be no longer 

 fatal. Indeed Pasteur originally procured immunity against 

 rabies by inoculating dogs from a series of monkeys and 

 rabbits, beginning with the monkey in which it was weakest, 

 and ending with the rabbit in which it was strongest. 

 Human small-pox when passed through the calf becomes 

 cow-pox, and when returned to man as vaccinia it protects 

 against the deadly variety of the disease. It is incredible 

 that, while unmodified rabies and small-pox produce toxins 

 which cause death, modified rabies and small-pox produce, 

 or cause to be produced, substances which are chemically 



