286 Dr. Edward E. Klein ' [Feb. 20, 



prepared animal, the microbe cannot grow and multiply, and cannot 

 therefore produce the disease. Pasteur's brilliant researches on 

 protective inoculation against hydrophobia are based on this principle. 



The same explanation applies also to diseases like scarlet fever, 

 anthrax, fowl cholera, swine fever, certain forms of septicaemia, in 

 which a first even mild attack is sufficient to protect the animal 

 against a second attack, however large the number, and however 

 great the virulence of the particular Bacteria introduced. 



For it must be obvious that it is practically the same, whether 

 the protective amount of the toxic substance is produced by the 

 Bacteria in the animal body, as is the case during a mild first attack 

 of the disease, or whether the protective amount of toxin is elaborated 

 outside the body, i. e. in an artificial culture, and is then introduced 

 into the animal body. In both instances the effect is the same, viz. 

 the animal body is hereby rendered capable of withstanding the 

 growth and multiplication of the particular Bacteria when a new 

 invasion takes place. 



What is the cause of this immunity or refractory condition ? 



In order to explain this, I wish first to draw your attention to the 

 familiar fact that different species of animals, and even different 

 individuals of the same species, offer a different degree of resistance 

 to the different infectious diseases. Whereas splenic fever or anthrax 

 is communicable to man, rodents, and herbivorous animals, it is only 

 with difficulty communicable to carnivorous animals or birds ; cholera 

 and typhoid fever are not communicable to any but man ; diphtheria 

 is communicable to the human species, to guinea-pigs, cats, and cows, 

 it is not communicable to some other animals ; tubercle or con- 

 sumption is communicable to man and herbivorous animals, in a less 

 degree to carnivorous animals, though these also take it but in a 

 smaller intensity ; certain other diseases are common to animals, but 

 are not communicable to the human species. 



If we inquire into the cause of this different susceptibility, we 

 find some very striking facts. Take anthrax : cold-blooded animals, 

 e. g. frogs, are unsusceptible as long as they are in their natural 

 conditions of temperature ; but if a frog be kept at the tempera- 

 ture of a warm-blooded animal, it is found susceptible to anthrax 

 (Petruschki). Birds are not susceptible to anthrax, but if its tem- 

 perature be lowered a few degrees it becomes susceptible to anthrax 

 (Pasteur). 



Or take another instance: rats are not susceptible to anthrax, 

 but if the animals be kept for some time under severe muscular 

 exercise, they become susceptible to the disease. Tame mice are 

 unsusceptible to glanders, but if phlorizin is administered to them 

 for some days, whereby a deposit of sugar takes phace in their tissues, 

 they become susceptible to glanders. The susceptibility and unsus- 

 ceptibility are expressed by saying that the living tissues in an 

 animal offer in the one case a favourable, in the other an unfavour- 

 able, soil for the growth and multiplication of the microbe, and that 



