OBSTACLES TO INFECTION 27 



possible. In the case of the diphtheria bacillus similar considerations 

 apply. It must be admitted, however, that mechanical injury is 

 of paramount importance, for we see that the tetanus bacillus, while 

 unable to grow and multiply in structures that are intact, can do so 

 when these have been previously or simultaneously bruised or lacer- 

 ated. For the reason that the two organisms in question can only 

 exist to advantage in damaged structures, Bail not inappropriately 

 speaks of them as necroparasites (necros dead). 



