Neuroanatomy Draw It To Know It

322 Neuroanatomy: Draw It to Know It

The Circle of Willis

Here, we will draw the circle of Willis, which Th omas
Willis fi rst described in 1664. Let’s stage our diagram
so that we learn the most commonly discussed arteries
fi rst and then add the more advanced vasculature. As
we learn the details of the cerebral vasculature, keep in
mind that two main arterial systems exist: an anterior-
lying internal carotid artery system and a posterior-lying
vertebral-basilar system. Start our diagram with the
anterior-lying internal carotid arteries; show axial sec-
tions through them. Next, show that the middle cerebral
arteries originate from the internal carotid arteries and
extend laterally. Th en, indicate that the anterior cerebral
arteries branch anteriorly from the internal carotids.
Next, at the bottom of the diagram, show that the two
vertebral arteries join to form the basilar artery, and then
show that the basilar artery ascends the brainstem. At its
apex, show that the basilar bifurcates into the posterolat-
erally directed posterior cerebral arteries. Now, show

that branches of the vertebral arteries combine inferi-

orly to form the anterior spinal artery. Next, connect

the internal carotid and posterior cerebral arteries

with the posterior communicating arteries. Th en, show

that the anterior communicating artery connects the

two anterior cerebral arteries. Next, on one side of

the diagram, we will draw the cerebellar vessels from

inferior to superior. First, draw the posterior inferior cer-

ebellar artery off the vertebral artery; then, the anterior

inferior cerebellar artery off the inferior portion of the

basilar artery; and lastly, the superior cerebellar artery off

the superior portion of the basilar artery. Now, draw the

lenticulostriate branches of the middle cerebral artery;

then, draw the anterior choroidal branch of the internal

carotid artery. Finally, draw the basilar perforators: the

pontine paramedian arteries, the short pontine circum-

ferential arteries, and the long pontine circumferential

arteries.^1 – 3

FIGURE 19-4 Magnetic resonance angiography of the cerebral arterial system demonstrating the circle of Willis.