Collateral circulation between external and internal carotid arteries

The facial, maxillary, and occipital branches of the external carotid artery can supply collateral blood flow to the intracerebral internal carotid artery branches when the extracerebral internal carotid artery is occluded.
Although the Circle of Willis is the more important source of intracerebral collateral flow from the contralateral internal carotid or vertebral arteries, external carotid collateral flow becomes more significant when multiple cervical vessels (carotid, vertebral) are occluded or in those patients with an incomplete Circle of Willis.