The brain is supplied by four arteries, the two common carotids and the vertebrals. On the right the common carotid usually arises from the brachio-cephalic trunk although on occasion it may arise directly from the aortic arch. On the left the common carotid usually arises directly off the arch of the aorta.
The common carotids ascend in the neck to the level of the thyroid cartilage where they bifurcate into external and internal branches. The external carotid supplies the tongue and the side of the head. The internal carotid has no named branches in the neck, it ascends to enter the skull at the external opening of the carotid canal. In the skull the internal carotid passes through the cavernous sinus and then gives off the opthalmic artery to the eye and divides into the anterior and middle cerebral arteries. The anterior cerebral artery sweeps forwards and backwards over the corpus callosum on the medial aspect of the cerebrum. The middle cerebral artery passes laterally to run in the space between the temporal lobe and adjacent lobes.
The vertebral arteries arise from the subclavian artery. They pass posteriorly and upwards to enter the vertebral canal passing from one cervical transverse process to the next. They enter the skull via the foramen magnum and sweep forwards to join one another to form the basilar artery anterior to the brain stem. At the base of the brain the basilar artery bifurcates to for the right and left posterior cerebral arteries.
The anterior communicating artery joins both anterior cerebral arteries and acts as a collateral channel between them. Similarly the posterior communicating artery joins the middle and posterior cerebral arteries to act as a collateral supply from one to the other. There is thus a circle of arteries at the base of the brain `Circle of Willis' that permit some collateral flow when flow in one or more of the vessels is reduced. Starting from the right anterior cerebral and moving anti-clockwise the circle consists of the anterior communicating artery, the left anterior cerebral, the left middle cerebral, the left posterior communicating artery, the left posterior cerebral artery, the right posterior cerebral artery, the right posterior communicating artery, the right middle cerebral artery and back to the right anterior cerebral artery. This circle is variable in its ability to compensate for poor flow in one of the vessels supplying it. Thus it is not unusual to encounter a patient with occlusion of one internal carotid artery with no evidence of a stroke in the territory of that artery.