The central branches supply the interior of the circle of Willis, more specifically, the Interpeduncular fossa.

The cortical branches are named for the area they supply and do not directly affect the circle of Willis.

Considerable anatomic variation exists in the circle of Willis, with classic anatomy seen only in about one-third of people.

[6] However, considering that the circle of Willis is present in many non-human species (reptiles, birds and mammals), and that arterial narrowing is mostly associated with old age and the human lifestyle, more generally applicable explanations of its functions have been suggested, such as dampening of pulse pressure waves within the brain[7] and involvement in forebrain sensing of water loss.

[1] The adaptive flow that the circle of Willis introduces can also lead to reduced cerebral perfusion.