PATHOGENESIS 55 
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 fistule originate in sterile cysts 
located on or adjacent to the second dorsal 
spine. 
2. The cause of typical fistulz is unknown, 
and the influence under which the cyst develops 
