PATHOGENESIS 39 
character, it gradually enlarges into a cyst of 
larger and larger dimensions until it becomes 
clinical by bulging at the surface of the base 
of the neck or at a point of least resistance 
adjacent thereto. The sac is at first very small 
and without any well developed limiting mem- 
brane but as it becomes larger the tissues react 
to the encroachment by forming a firm con- 
nective tissue layer around it and at the same 
time fibrin and cells attach themselves to the 
inner wall and thus pave it with a smooth 
carpet that completes a firm encapsulation of 
the liquid contents. The amount of connective 
tissue contained in the encapsulating structure 
varies with the chronicity of the process. In 
old, slowly forming cases the fibrous elements 
may form in large quantities, encroaching upon 
the capacity of the sacinternally and gradually 
bulging exteriorly until the lesion is in fact 
more fibrous than cystic. Thus we have the 
large fistulae of the withers in which fibrosis is 
the dominating feature. This sac is uniformly 
related to one of the dorsal spines, usually the 
second, but sometimes the third and in rare 
cases the fourth or even the fifth. We have 
never seen a typical fistula of the withers orig- 
inate farther back. Those located on the sum- 
mit or further back are always of the other 
variety (traumatic or atypical). The point of 
