166 IMMUNITY. 



fection with the bacterium of this disease has not been established, 

 although frequently tried, in any of the animals upon which experi- 

 ments have been made. Leprosy, scarlet fever, yellow fever and 

 measles are other diseases which inflict themselves upon mankind, 

 but to which the lower animals are apparently insusceptible. Some 

 infections are prevalent among certain species of animals, while upon 

 others they seem to be without effect. For instance, anthrax is 

 common among cattle and sheep, while many carnivorous animals 

 wholly escape this infection. It is an interesting fact that in na- 

 ture some animals, such as the guinea-pig and rabbit, are not known 

 to suffer from epidemics of anthrax, but this disease can be easily in- 

 duced in these animals by artificial inoculation. Toxin and bacterial 

 immunity are sometimes, but not always, possessed by the same 

 animal. Eats and mice are insusceptible to both the bacUlus and 

 the toxin of diphtheria, but, as we have already seen, many animals 

 succumb to the typhoid toxin, while none are susceptible to this 

 infection. 



One of the important factors in natural immunity lies in the fact 

 that the toxicogenic germ is unable to multiply in the animal body. 

 The bacillus pyocyaneus is frequently found upon the surface of 

 man's body, especially in the axillary and inguinal regions, and 

 sometimes it occurs in the intestines, but notwithstanding this al- 

 most constant proximity of this organism man is but rarely injured 

 by it, yet, nevertheless, this bacillus produces a toxin to which man 

 is susceptible. The micrococcus prodigiosus is not classed among 

 the pathogenic bacteria, and yet experimentation has shown that its 

 toxin, which has been used in the treatment of malignant growths, 

 has a marked effect upon man. It is probable that most of the sa- 

 prophytic bacteria contain intracellular substances which when re- 

 peatedly injected into animals in relatively large amounts are capable 

 of causing death. The sarcines are usually regarded as altogether 

 harmless, but it has been shown that the intra-peritoneal injection of 

 the cellular elements of these organisms is followed by death. It 

 has been found that large amounts of the spores of the tetanus bacil- 

 lus, when completely freed from the toxin, may be injected into rab- 

 bits and guinea-pigs with no further injury than the formation of a 

 pocket of pus at the place of injection, but these animals are highly 

 susceptible to tetanus toxin, which causes in them typical symptoms 

 of the disease and induces speedy death. The natural immunity 

 possessed by some animals to tetanus infection may be overcome by 

 the employment of mixed cultures, even when the accompanying 

 microorganism is apparently wholly without effect. A like result 

 may be secured by the simultaneous introduction into the animal 

 body of certain chemicals, such as lactic acid. On the other hand, 

 certain animals are quite insusceptible to the toxin, and at the same 

 time easily infected with the microorganism. A healthy man is rela- 



