Anastomoses were accurately localized by
using vessel branch angles as determinants.
Branch angles of two joined arterioles having
MCA source for one and ACA source for the
other characteristically face each other (Fig. 2).
Because artrial blood normally flows into smaller
vessels distal to a branch point, not combined
into one larger vessel (unless flow direction
is reversed), one can identify, on the basis
of angle, if supply is from the ACA or MCA.
Either the narrowest vessel width or the vessel
distance one-half way between the two branch
points was selected as the anastomosis point.
The latter criterion was applied in cases where
a clear-cut narrowing was not observed.
With origin at the branch point and termination
at the anastomosis, a collateral is part of a
pattern that is dependent upon the number of
interarterial collaterals stemming from the
branch point. A branch point was never observed
to be the origin of more than two arterioles,
one or both of which were interarterial
collaterals. The most evident pattern was
where both branches were end rami that joined
a nearly mirror image set. The shape of the endto-end
vascular complex is roughly that of a
diamond (Fig. 2A, C). Common variations of