THE SPORE-BEARING RODS 359 



the nerves. The symptoms of the disease are not apparent 

 until the nerve cells have been injured as a result of the 

 absorbed toxin. These injured cells stimulate the muscles 

 (particularly the muscles about the area infected) to spas- 

 modic contraction (tetanus). In man and some animals 

 the nerve centers governing the muscles of the face are par- 

 ticularly apt to be affected and the contraction of the 

 muscles causes the characteristic " locking " of the jaw. 



It is apparent that an incubation period is necessary 

 between the inoculation of organisms into the wound and 

 the development of the symptoms. This is the time neces- 

 sary for the formation of the toxin and the injury to the 

 central nerve cells. In man this incubation period averages 

 about 9 or 10 days, in horse it varies from 4 to 20 days or 

 even more. 



Immunity. The characteristic toxin of the tetanus 

 organism may readily be prepared in broth in which it is 

 grown under anaerobic conditions. In commercial manu- 

 facture of antitoxin, this toxin is first titrated by deter- 

 mining the smallest amount which will certainly kill a 350 

 gram guinea pig in 2 to 4 days. This is called the unit of 

 toxicity. At first small and then increasing doses of 

 toxin are injected at suitable intervals into the horse; 

 eventually the blood serum will be found to contain the 

 specific antitoxin in considerable quantities. The animal is 

 bled and the serum obtained in much the same fashion as 

 has been previously discussed for diphtheria antitoxin. 



The tetanus antitoxin is an excellent prophylactic, that is, 

 when injected soon after the organism has been introduced 

 into the body, it will neutralize the poison as it is formed 

 and prevent the development of the symptoms of tetanus. 

 When once, however, the symptoms have developed, the 

 antitoxin is not always able to prevent a fatal result. In 

 other words, antitoxin cannot undo the damage already 

 done to the central nerve cells by the toxin taken up. It 



