Saccular Aneurysms

Why do saccular aneurysms happen at branching points in the Circle of Willis? At these branching points, the wall is weak because the vessel lacks a media layer. Where is the most common site for these berry aneurysms? At the point when the anterior communicating branches from the anterior cerebral artery. You'll probably find something when you do a spinal tap. ;)

Dorsal Columns, Spinothalamic, Corticospinal Tracts