dunno if some of  you might be interested or not... just following up my obsession with wanting to know and understand everything about this mm thing...

some websites about the arteries affected etc..... that i've found good:

http://www.csuchico.edu/~pmccaff/syllabi/CMSD%20320/362unit11.html

http://www.csuchico.edu/~pmccaff/syllabi/CMSD%20320/336unit4.html

although somewhat confusing at times....

and this paragraph here (cause I personally just really didn't get it):

The blood supply to the brain is from four main blood vessels – two vertebral arteries and two carotid arteries. The vertebral arteries enter the skull from the backbone and mainly supply the brain stem and cerebellum, whereas the two carotid arteries enter the skull from the front of the neck and mainly supply the two cerebral hemispheres.
All four arteries join up in a rough circle, which helps to maintain an adequate supply of blood if one artery gets blocked. Water mains and electricity supplies operate on similar principles, to try to maintain adequate supplies even if part of the supply breaks down. Unfortunately, the effectiveness of the circle of arteries varies from person to person and often does not protect people from the symptoms of a stroke if one of the main arteries becomes blocked.