During a C1-C2 posterior spinal fusion, while performing the dissection of posterior arch of C1 and pedicle of C2, the surgeon encounters rapid pulsatile arterial bleeding. The artery is tamponaded off,vascular surgery is consulted and an intra-operative angiogram is performed. Given adequate collateral flow, the injured artery is embolized. Post-operatively the patient's neurologic function is normal. Injury to this arterial structure could have best been avoided by minimizing dissection in which of the following areas?