118 FISTULA AND POLL-EVIL 
tacked from without. Its dorsal face is at- 
tack first, while in the other, the under part 
at the atlantal bursa, is attacked before the 
upper part becomes involved. 
This form of fistula can only be recognized 
as such at the early stage, for once spread 
deeply into the poll it has no differentiating 
qualities. 
Etiology 
The cause of poll-evil, like that of fistula of 
the withers, is by no means settled. The aca- 
demic explanation (traumatism) that seems to 
have satisfied all past writers on the subject 
entirely lacks confirmation. Wherever this 
subject is discussed verbally or in literature the 
matter of etiology is always indifferently 
passed over as if it is a settled fact that poll- 
evil is a traumatism. No one seems to have 
ever questioned seriously the theory of trau- 
matic origin in spite of the fact that no one 
seems to have ever actually traced a typical 
poll-evil to any definite traumatism. The 
truth seems to be that traumatism plays no 
part whatever in its cause. Except in the rare 
case that originates with a wound of the poll 
it is plain that we must seek farther for a rea- 
sonable theory of cause. It is our opinion that 
the cause is unknown, that the serous sac that 
starts it develops under influences we cannot 
