PATHOGENESIS 65 



the same as during the abscess formation of the 

 typical form. There is, however, never as much 

 fibrous tissue formed as in the other variety. 

 Although the environs of the tracts and ab- 

 scess are surrounded by fibrous tissue it is 

 never so abundant. Voluminous fibrosis is an 

 attribute only of slowly forming typical fistula. 



The secondary pus cavities of atypical fis- 

 tula are more superficial than those of the 

 typical kind, particularly during the early 

 stages. These are often subcutaneous, some- 

 times just under the superficial fascia and at 

 other times simply under the trapezius, but 

 later when the disease has become more deeply 

 seated and has traveled anteriorly along the 

 ligament, they may be found in the same deep 

 locations as in the typical forms. On the 

 other hand, it might be said here that these 

 superficial abscesses do not occur except in the 

 atypical ones, except when lancing or other 

 improperly directed treatment causes them. 



The differences pathologically between these 

 two forms might be summarized as follows : 



1. Typical fistulcC originate in sterile cysts 

 located on or adjacent to the second dorsal 

 spine. 



2. The cause of typical fistula is unknown, 

 and the influence under which the cyst develops 



