THE BACILLUS OF TETANUS. 401 
nor is any other neutralized by tetanus antitoxin. The 
other characteristics also of this bacillus are usually dis- 
tinctive, though microscopical examination alone cannot 
be depended on to make a differential diagnosis. Diffi- 
culty arises when other anaérobic or aérobic bacilli, 
almost morphologically identical with the tetanus 
bacillus, are encountered which are non-pathogenic, 
such as the bacillus pseudotetanicus anaérobius, already 
mentioned, and the bacillus pseudotetanicus aérobius. 
It is possible, however, that both these bacilli, when 
characteristic in cultures, are only varieties of the 
tetanus bacillus, which, under unfavorable conditions of 
growth, have lost their virulence. These non-virulent 
types do not, as a rule, have spores absolutely at their 
ends, and the spores themselves are usually more ovoid 
than tho:e in the true tetanus bacilli. 
26 
