CHEMICAL CEPTORS 105 



neutralize the toxin of tuberculosis. The antitoxin 

 of diphtheria is inactive against typhoid, erysipelas or 

 pneumonia. As Ehrlich has picturesquely expressed 

 it, the anti-bodies produced by the blood substance 

 in response to infectious invasions are " charmed bul- 

 lets" which strike only the objects against which they 

 have been evolved as weapons by the organism. 



Thus we see that the same law of natural selection, 

 which has built up a system of contact ceptors in the 

 skin for the defense of the body against gross material 

 enemies, has evolved a system of chemical ceptors and 

 specific chemical reactions for the apprehension and 

 destruction of microscopic enemies within the organism. 

 The same law of phylogenetic association, which is 

 responsible for the muscular reaction of defense against 

 physical injury, is responsible for the chemical defense 

 against microorganisms, and against all foreign protein 

 material whether from excessive protein food, pregnancy 

 or infection. As a physical blow, light, heat, cold or 

 tickling supplies the needed factor for the release of 

 energy for physical defense through the excitation of 

 contact ceptors, so the injection of live or dead bacteria 

 or of foreign protein substance into the blood stream 

 of the body supplies the exciting factor which calls 

 out the activity of chemical defenses through the ex- 

 citation of chemical ceptors. On this important fact 

 is based the theory and practice of serum-therapy 

 and of vaccination, by which diphtheria, typhoid, 

 smallpox, tetanus and other diseases are conquered. 



The physician who contemplates this identity of 

 nod association in the infections with the nod asso- 

 dations of physical contact will call to mind many 



