t
condition of the vessels in two cases of femoral arterio-venous aneurism
was, that in either case a clean perforation existed.

It is improbable that notching of the two vessels can primarily produce
a pure varix, although it may result in the formation of an
arterio-venous aneurism, especially if the bullet should have passed
between the two vessels in such a way as to notch the contiguous sides.
It is impossible to say, in any given case, what the result of secondary
contraction of a sac produced in this manner may be in the determination
of the ultimate relation of the vessels. In many of the cases clinically
designated pure varix, the remains of such a sac may still actually
persist. In the case also of pure perforation of the vessels, it is
difficult to believe that a localised blood cavity has not originally
existed. Given complete division of the vessels, as far as my experience
went, arterial haematoma was the uniform result.

Under these circumstances I am inclined to believe that a symmetrical
perforation of both vessels is the most common precursor of either
condition; that the pure varix is the rarer and less likely result, and
that its formation is dependent mainly on certain anatomical conditions.
The most important of these conditions are the proximity and degree of
cohesion of the two vessels, the comparative spaciousness or the
opposite of the vascular cleft, and the degree of support afforded by
surrounding structures.

Thus, the close proximity of the popliteal artery and vein, together
with the particularly firm adhesion which exists between the vessels,
probably favours the formation of a varix; again, a varix more readily
forms if the femoral artery and vein are wounded in Hunter's canal than
if the injury is situated high in Scarpa's triangle, where the vessels
lie in a large areolar space. The passage of a bullet between an artery
and vein may perhaps produce either condition, but wide separation of
the two vessels, as for instance of the subclavian artery and vein,
renders an aneurismal sac almost a certainty. These suggestions seem
borne out by the cases recounted below, since the pure varices are one
femoral, one popliteal, and one axillary. I cannot include the calf and
forearm cases, as the existence of a small sac could not be disproved.

To these anatomical factors certain others must be added. In most cases
a false sac exists at first, which tends to undergo contraction and
spontane