Types and Subtypes of the Posterior Part of the Cerebral arterial Circle in Human Adult Cadavers

The main cerebral distribution center of 15-20% blood flow from the cardiac output is the cerebral arterial circle or circle of Willis, a nonagon of collateral vessels on the human brain base. At about 4 mm stage of the embryo, the primitive internal carotid arteries, which develop as cranial extensions of the paired dorsal aorta, are formed. Paired longitudinal neural arteries appear along the hindbrain and coalesce to form the basilar trunk at the 7to 12-mm stage. The caudal division of the primitive internal carotid artery anastomoses with ipsilateral neural artery and becomes the posterior communicating artery. At the 40-mm stage the posterior cerebral arteries are as extensions of the posterior communicating arteries. The vertebrobasilar system develops and thus participates in the supply of the posterior cerebral artery through the segment between the basilar artery and the post-communicating part of the posterior cerebral artery. In that phase, the component vessels of the circle of Willis all have the same caliber (Silver & Wilkins, 1991). The posterior cerebral artery originates from the basilar bifurcation within the interpeduncular cistern. From its origin the artery curves superior to the corresponding oculomotor nerve in relation to the antero-medial portion of the cerebral peduncle and joins ipsilateral posterior communicating artery (Yasargil, 1984). The posterior communicating artery takes origin from the infero-lateral wall of the cerebral part of the internal carotid artery. It is encased in a sleeve of arachnoid along the course from the carotid cistern to the piercing of the interpeduncular cistern and junction with posterior cerebral artery (Yasargil, 1984). Topographically, the circle of Willis is divided on anterior and posterior parts. The anterior part composes five vascular components – bilateral cerebral parts of internal carotid arteries (communicating and choroid subparts) and the pre-communicating part of anterior cerebral arteries interconnected by the anterior communicating artery; the posterior part composes