The circle of Willis is a confluence of vessels that gives rise to all of the major cerebral arteries. It is supplied by the paired internal carotid arteries and the basilar artery. When the circle is complete, it contains a posterior communicating artery on each side and an anterior communicating artery. The circle of Willis shows many variations among individuals. The posterior communicating arteries may be large on one or both sides (embryonic type); the posterior cerebral artery may be thin in its first stretch (embryonic type); and the anterior communicating artery may be absent, double, or thin. Despite these variations, usually the arteries are fairly consistent.
The course of the large arteries is largely ventral to the brain in a relatively small region. The arteries course in the subarachnoid space, before entering the brain itself. Each major artery supplies a certain territory, separated by border zones from other territories; sudden blockage in a vessel affects its area immediately, sometimes irreversibly.
The arterial blood for the brain enters the cranial cavity by way of two pairs of large vessels: the internal carotid arteries, which branch off the common carotids, and the vertebral arteries, which arise from the subclavian arteries. The vertebral arterial system supplies the brain stem, cerebellum, occipital lobe, and parts of the thalamus, and the carotids normally supply the remainder of the forebrain. The carotids are interconnected via the anterior cerebral arteries and the anterior communicating artery; the carotids are also connected to the posterior cerebral arteries of the vertebral system by way of two posterior communicating arteries, part of the circle of Willis.
After passing through the foramen magnum in the base of the skull, the two vertebral arteries form a single midline vessel, the basilar artery. This vessel terminates and bifurcates as the left and right posterior cerebral arteries. These may be thin, large, or asymmetric depending on the embryonic pattern.
These branches make up a very complex and unique system that is worht knowing about. Naturally, the next topics will be on problems or pathologies that occur within the circle. There can be many disorders that can occur because of either anatomical differences between people or sheer trauma to a certain area. Examples follow. Occlusive cerebrovascular disorder can result from arterial or venous thrombosis, or embolism, and can lead to infarction of well-defined parts of the brain. Because each artery irrigates a specific part of the brain, it is often possible, on the basis of the neurologic deficit, to identify the vessel that is occluded.
Waxman SG, "Chapter 12. Vascular Supply of the Brain" (Chapter). Waxman SG: Clinical Neuroanatomy, 26e: http://www.accessmedicine.com/content.aspx?aID=5273762.
Barrett KE, Barman SM, Boitano S, Brooks H, "Chapter 34. Circulation through Special Regions" (Chapter). Barrett KE, Barman SM, Boitano S, Brooks H: Ganong's Review of Medical Physiology, 23e: http://www.accessmedicine.com/content.aspx?aID=5245769.