Circle of Willis Order

Mnemonic

Trace it as “two in front, two behind, joined by three communicators”:

• Anterior: the two anterior cerebral arteries, joined by the anterior communicating artery
• Middle: the internal carotid arteries give the middle cerebral arteries
• Posterior: the two vertebral arteries join as the basilar, which divides into the two posterior cerebral arteries
• The posterior communicating arteries join front to back on each side

Aneurysm sites in order of frequency: anterior communicating (about 35 per cent), posterior communicating (30 per cent), middle cerebral bifurcation (20 per cent).

Each has a signature: a posterior communicating aneurysm compresses the third nerve, giving a painful pupil-involving third nerve palsy; an anterior communicating aneurysm can compress the optic chiasm.

The circle is complete in fewer than half of people, which is why collateral supply is so variable and why occlusion is tolerated by some and not others.

Expansion

• Anterior cerebral arteries joined by the anterior communicating artery
• Posterior cerebral arteries joined to the internal carotids by the posterior communicating arteries