44 CLINICAL BACTERIOLOGY. 



ent effects in the same animal in accordance with the site 

 of introduction. An amount of pneumococci that, injected 

 subcutaneously, would cause death in a dog, will, when 

 given by intraabdominal injection, cause no disturbance. 

 Conversely, cholera-bacilli act more energetically in guinea- 

 pigs when introduced into the peritoneal cavity than by 

 subcutaneous inoculation. Cattle tolerate without ill result 

 the bacilli of symptomatic anthrax when injected into 

 a vein, whereas the same material introduced subcuta- 

 neously invariably gives rise to disease. In the same way 

 also in human pathology the point of entrance of the bac- 

 teria into the body is of importance for the occurrence of 

 an infection. Thus, cholera-infection occurs, as a rule, only 

 through the intestine, pneumonic infection only through the 

 upper air-passages at least, it has been demonstrated that 

 the subcutaneous injection of not too large amounts of 

 cholera-bacilli or of pneumococci is without injurious effect 

 upon human beings. 



(d) The Susceptibility of the Infected Organism (Pre- 

 disposition). The susceptibility of different species of 

 animals to an infectious disease varies widely. To tetanus, 

 for instance, the guinea-pig and the white mouse are highly 

 susceptible, the rabbit far less so, and fowl so little sus- 

 ceptible that it is only with difficulty that tetanus can be 

 induced in these animals. To no variety of bacteria are all 

 animals equally susceptible. Thus, while cattle, mice, and 

 guinea-pigs are highly susceptible to anthrax, rats, dogs, 

 and birds are almost entirely insusceptible, and cold- 

 blooded animals tolerate the pathogenic microorganisms 

 almost universally without injury. 



Also in the same animal species differences in suscepti- 

 bility exist toward the same bacterium. Thus, field-mice 

 suffer from glanders, while white mice do not. Older 

 animals are, in general, less readily infected that is, they 

 are less susceptible than young animals. Congenital sus- 

 ceptibility is designated natural predisposition. This pre- 

 disposition is, however, not constant in degree even in the 

 same animal. It may be intensified or diminished. Thus, 

 insusceptible animals may be rendered temporarily sus- 

 ceptible to certain diseases by protracted hunger, great 

 muscular exercise, and similar influences. Such a tempo- 

 rary predisposition can, for instance, be induced in frogs to 

 anthrax by exposure to heat, in fowl to the same disease 



