Blood supply to the brain (circle of Willis)

The anatomy of the blood supply to the brain, including the circle of Willis (or circulus arteriosus). I talk about the vertebral and internal carotid arteries, the basilar artery and the circle of Willis, and mention the cerebral and cerebellar arteries, the blood-brain barrier and the idea of cerebral perfusion pressure. See also this online thing Steve Atherton and I made: Music by Broke for Free (Licensed for use under Creative Commons Licenses, see above links for details).