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Apart from the circle of Willis there is a plethora of extra-cranial collateral channels that augment cerebral perfusion during individual clamping of branch vessels. These include collateral channels between (A) the external and internal carotid arteries; (B) the right and the left carotid arteries; (C) the upper and lower body; and (D) the subclavian and carotid arteries [Adapted from (21) and reproduced with permission from Elsevier]
