In rat, the unpaired ACA courses in the interhemispheric
fissure with branches distributing
bilaterally to the medial cortex on the
dorsal aspect of the brain. The bilateral MCA
is positioned laterally to supply the greatest
portion of the neocortex. The ACA-MCA collaterals
are located at the interface of distal
branches of these two major supply vessels.
Collateral units are present at the vascular
border zone extending across frontal, parietal,
and into occipital regions (Fig. 3). The parasagittal
locations of the anastomoses vary
from frontal to occipital fields. Junctional deviations
from the midline are greatest near the
frontal pole and least in the occipital region.
Numerous branches of the posterior cerebral
artery (unlabeled, Fig. 3) join rami of the MCA,
but not the ACA, along a course roughly parallel
to the transverse fissure separating occipital
and cerebellar structures. The PCA-MCA
anastomoses are intermediate in position between
the dorsal ACA-MCA junctions and the
ventral anastomoses of the arterial circle of
Willis. Because of position, the majority of
PCA-MCA collaterals may fill before those of
the dorsal collaterals. Most PCA-MCA collaterals
are not observable in our dorsal view
preparation, so the filling pattern could not be
seen.