CHEMICAL CEPTORS 97 



proteins, infection, auto-intoxication and the toxins of 

 pregnancy. Invading bacteria and the chemical prod- 

 ucts of their activity are adequate stimuli to certain 

 chemical ceptors. These mechanisms, when excited 

 to intense activity as in infection, cause a breaking 

 down of foreign protein molecules without breaking 

 down the living protein molecules of the organism ; 

 and by this means the standard of chemical purity of 

 the body is maintained in the presence of infection. 

 This chemical defense gives rise to the principal phe- 

 nomena of infection and auto-intoxication and will be 

 discussed later. 



In the response to local infection by phagocytosis 

 and the response to general infection by processes of 

 immunity we have recapitulations of phylogenetic 

 associations of a vast antiquity. 



Retracing our steps down the line of the animal 

 kingdom, we note that the chief difference between 

 animals and vegetables is the inability of the former 

 to nourish themselves directly from the environmental 

 media. Animals cannot transform the elements of 

 inorganic salts into protein molecules, but must de- 

 pend upon a previous synthesis and assimilation of 

 those salts by plants or lower animals. This weak- 

 ness is exhibited in the single-celled amoeba and other 

 protozoa by a major diet of bacteria, almost any species 

 of which they will envelop and attempt to appropriate. 

 Against this offensive activity on the part of the pro- 

 tozoa the bacteria have developed a counter self- 

 defensive adaptation in .the form of a poisonous secre- 

 tion, which kills their enemy after it has devoured 

 them. This war between "host" and " parasite," 



