The only weapon with which the unconscious patient can immediately retaliate upon the incompetent surgeon is hemorrhage -William S Halsted.

Also known as loop of Willis,Willis polygon and cerebral arterial circle.

Blood vessels supplying the brain consists of 2 separate vascular systems:

• Internal carotid arteries
• Vertebral arteries

In the base of the brain both systems are connected bilaterally by posterior communicating arteries and left to right side by anterior communicating arteries forming the “Circle of Willis”(COW).

Other key points:

• In subclavian steal syndrome ,blood is “stolen” from COW to preserve blood flow to upper limb.
• This syndrome results from a proximal stenosis of the subclavian artery.
• Variations in the shape of COW is also seen in mentally ill and migraine patients which include hypoplasia of component vessels,absence of posterior communicating arteries,duplication/triplication of anterior communicating arteries,etc.

Mnemonics:

Cell is Clearly Circulating

C-Cortical branches

C-Central branches

C-Choroidal branches

Sources: BD Chaurasia’s Human anatomy 7th edition-volume 4

Slideshare.net -Circle of Willis by Luiz Roberto Meier Update 2