DISEASE, INFECTION, AND RESISTANCE 283 



will cause the body to become accustomed to its presence 

 and to endure relatively large amounts of the material. 

 This resistance, however, is not due to the presence or for- 

 mation of substances which definitely neutralize the activity 

 of the morphine. 



The exact chemical constitution of none of the toxins has 

 thus far been determined. Generally when injected into 

 an animal they require a period of incubation before the 

 results of the poisoning action are manifest. In general 

 they injure the body probably by combining chemically 

 with certain of the body cells or tissues. The toxin pro- 

 duced, for example, by the bacillus causing tetanus or lock- 

 jaw combines with the central nerve cells, injuring them 

 and producing thereby the symptoms of the disease. 



Sources of Toxins. — ^A relatively small number of the 

 disease-producing bacteria develop toxin. The most im- 

 portant of these are the bacteria which produce the diseases 

 diphtheria, tetanus and botulism (one type of food poison- 

 ing) . The organisms causing blackleg in cattle and gaseous 

 gangrene in man and certain types of dysentery bacilli 

 develop toxins likewise. It is evident, inasmuch as 

 toxins are produced by only few of the pathogenic bacteria, 

 that antitoxins for immunizing against or curing such 

 diseases must be limited in number. There is no probability, 

 for example, that an antitoxin can ever be secured for a 

 disease such as tuberculosis in which there is no evidence of 

 any toxin production by the bacteria. 



These toxins are excretions of the bacterial cells. Certain 

 other plant cells are also known which can produce toxins. 

 The juice of the castor oil bean seed, the inner bark of the 

 black locust and the seed of the jequirity bean, all contain 

 toxins. Certain poisonous animals are also toxin producers. 

 Snake venom, for example, contains one or more toxins. 

 The stings of many insects, fish and other animals are 

 injurious because of the presence of true toxins. 



