THE ANIMAL ORGANISM. 67 



But artificial means can be used, such as the use of chemical 

 agents added to the nutrient soil, or by passing the germ through 

 animals who are in some sense immune, and are less affected than 

 the animals for whom it is strictly pathogenic. 



Thus the bacillus of swine-erysipelas, which is quite virulent 

 for pigs, when passed through rabbits loses much of its power, 

 and again introduced into pigs will sicken them but slightly. 

 Sunlight or any other agent that is destructive to germs will 

 also weaken them when used cautiously. 



Heat is the surest agent to lessen the action. 



The longer it takes to produce the attenuation, the more lasting it is. 



The grade of virulence will oftentimes remain through suc- 

 cessive generations. 



ISoiLe of the attenuations have been named according to the 

 animal that they will affect ; thus, Mice-anthrax is a culture of 

 anthrax which has been exposed to a temperature of 42.6 C. for 

 twenty days, and which will destroy nothing larger than mice. 

 A culture exposed for ten days will kill nothing larger than 

 rabbits, etc. 



The only explanation that can be given of attenuation is that 

 the microbes, though similar in appearance, differ, in that the 

 weaker ones give rise to less toxic products ; they have been 

 exhausted. 



Nageli makes use of the simili of the sweet and bitter almond, 

 the one poisonous, because it contains amygdalin, but both pos- 

 sible to be borne on the same tree, and looking alike in every 

 particular. 



The Resistance of the Animal Organism to Bacteria. The 

 body is in some sense resistant to bacteria ; to some more, to 

 others less ; and this resistance has been variously explained. 



Chemical Theory. The greater or less alkalinity of the blood di- 

 minishing or increasing the virulence is the explanation of some. 



The Theory of the Action of the Serum of the Blood. It has 

 been lately shown that the serum of the blood has a direct in- 

 hibiting action on all bacteria ; and this is directly dependent 

 upon the quantity and quality of albumen in it. It was for- 

 merly thought the salts of the blood were the main factors, but 

 these only serve to keep the albumen in good condition. 



