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 poll-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- 



