IMMUNITY. 233 



environment in which it can grow and produce its toxin. 

 Such mixed infections probably account for those cases where 

 the tetanus does not manifest itself until some time after the 

 injury has occurred. The bacilli die in the presence of oxy- 

 gen, but the spores survive, although they remain inactive 

 until such time when conditions are favorable for their devel- 

 opment. If the infection has occurred in an open wound, 

 the disease may not manifest itself until after the wound has 

 closed. 



The tetanus bacillus is an extremely toxic germ, and the 

 manifestations of the disease are entirely due to absorption of 

 the toxin. 



The bacillus does not spread throughout the body, but 

 remains confined to the point of injury. It is never found 

 in the tissues of the body, nor in the blood unless injected 

 experimentally, y^j- of a milligram of a filtered culture 

 is sufficient to kill a mouse. 



A man weighing 175 pounds would be killed by 0.23 of 

 a milligram of tetanus toxin. 



This toxin is very easily destroyed ; and its chemical com- 

 position is unknown. 



It is produced in large quantities by the bacillus, and its 

 action is very rapid. 



It first affects the muscles nearest to the point of infec- 

 tion, and then, in order, all the parts of the body. Strychnine- 

 poisoning resembles tetanus in its manifestations ; and, like 

 strychnine, the tetanus toxin has a predilective action on the 

 spinal cord. The action of the tetanus toxin is localized in 

 the spinal cord. That some of the toxin is taken up by the 

 blood and lymph-currents has been proved by injecting blood 

 from a case of tetanus into an animal, which promptly devel- 

 oped the disease. The toxin is excreted by the kidneys. 



Immunity : In view of the fact that the bacillus remains 

 localized at the site of infection, and that the circulating 

 toxin is responsible for the manifestations of the disease, 

 immunity to tetanus must be against the toxin and not the 

 germ. The serum of animals immunized to tetanus confers 

 immunity, and it also exerts a curative action ; but for cura- 

 tive purposes a larger amount of serum, or a more active 



