PATHOGENESIS 
Poll-evil, like fistula of the withers, we di- 
vide into two classes: typical and atypical. By 
typical poll-evil we mean those cases that de- 
velop independent of any known cause in the 
atlantoid bursa, just as fistula of the withers so 
frequently develops in the dorsal bursa. By 
atypical poll-evil—a rare entity—we designate 
those that originate from abrasions or wounds. 
The former invade outward while the latter in- 
-vade inward from the initial seat. 
The course of all typical poli-evil we divide 
into three stages: the saccular stage, the phleg- 
monous stage and the fistulous stage, as in fis- 
tula of the withers. 
The saccular stage begins in the form of a 
distension of the atlantoid bursa with a sero- 
fibrinous fluid that is sterile and whose influ- 
ence causes the formation of a well defined 
membranous wall composed of a cellular inter- 
nal face fortified by fibrous tissue. As the les- 
ion becomes older the fibrous tissue becomes 
more abundant, sometimes amounting to an 
extensive fibrosis of the surrounding muscula- 
ture. The sac may bulge on one or both sides 
and may be small or large enough to conspicu- 
