Circle of WillisCircle of Willis((arterial circle, circulus arteriosusarterial circle, circulus arteriosus))

The arterial circle, described by Thomas Willis in 1664, is an important polygonal anastomosis between the four arteries that supply the brain: the two vertebral and the two internal carotid arteries.

It is formed by the posterior cerebral, posterior communicating, internal carotid, anterior cerebral and anterior communicating arteries.

The circle forms an important means of collateral circulation in the event of obstruction of a major vessel. Variations in the size of the vessels that constitute the circulus are very common.

Angiographic view

Carotid angiogramCarotid angiogram

Carotid siphon

Vertebral angiogramVertebral angiogram

Showing the origins of main arteries

circle of Willis in situ