Shift your attention from brain structures to the arterial supply of the brain and in particular the cerebral arterial circle (of Willis) . Using a whole brain, turn the specimen upside down and identify the various arteries that supply the arterial circle as well as the components of the circle itself. (Figure 7-4)

Begin by locating the two vertebral arteries lying on the anterior surface of the medulla. Near the caudal (inferior) end of the pons, the two vertebral arteries unite to form the basilar artery. The basilar artery lies on the anterior surface of the pons in a shallow groove. Near the cranial (superior) end of the pons, the basilar artery splits into two posterior cerebral arteries. These arteries immediately course laterally and posteriorly to reach the posterior aspects of the cerebrum.

Now identify the two internal carotid arteries. These will only be stumps, cut during the brain's removal. You can find them just lateral to the infundibulum and posterior to the optic chiasm. Each of these arteries give rise to a large laterally running middle cerebral artery that disappears under the anterior portion of the temporal lobe and a smaller anterior cerebral artery that runs anteriorly under the stubs of the optic nerves as it heads to the anterior and medial surface of the cerebrum. We still need to find the rest of the arteries that actually complete the arterial anastomotic circle.

The two posterior communicating arteries connect the posterior cerebral arteries with the internal carotid arteries. These are usually small arteries and variable in size even from side to side. The last artery completing the arterial circle is the anterior communicating artery. This small artery connects the two anterior cerebral arteries; often hidden either under the optic tracts or optic chiasm. This completes the set of arteries that form the cerebral arterial circle (of Willis) .