Today at Vascular surgery (VA), i got pimped on the circle of willis.  By now, i should know it backwards and forwards but I had seriously not looked at that thing for a while.  I could only recall 1/3 of what was asked of me... pretty crappy and annoying.  But the residents are really cool and teach well.

So anyhow, this is a picture of the circle of willis:

The internal carotid goes all the way up to the brain and gives off branches of the anterior cerebral and posterior communicating arteries (both part of the circle).  The internal carotid also gives off the anterior choroidal and ophthalmic arteries (the last branch).   As far as the circle of willis is concerned, the internal carotids supply the anterior.

The posterior part of the circle is supplied by the basilar artery (which itself is formed when the vertebral arteries join together).   Vertebal arteries arise from the subclavian arteries.  The patient that we saw actually had the left vertebral coming right of the aortic arch.
Anywho, the basilar artery ultimately supplies the posterior communicating and the posterior cerebral (before it gives off branches that supply the cerebellum and the anterior spinal artery that supplies the spinal cord).