The walls of the cerebral vessels are thin, and both the middle and outer membranes are thinner than those of the extracranial arteries of the same diameter. The internal carotid artery and the vertebral artery enter the skull through the skull base and first form an arterial ring (Willis ring) at the base of the brain, from which branches are sent into the brain, all of which are distributed radially from the skull base toward the ventricles. The arterial ring supplying the cerebral cortex repeatedly branches on the cortical surface to form the plexus of small soft membrane arteries, and then the plexus sends out cortical and medullary arteries deep into the brain parenchyma. The penetrating arteries (central branches) supplying the deep brain pass from the anterior penetrating mass of the brain base and the pedunculopontine fossa into the brain from the bottom up, and between the penetrating arteries, although there are abundant vascular anastomoses, the anastomosing branches are small, and the regulation and compensatory capacity of cerebral blood flow is weak. The density of capillaries in the gray matter of the brain is richer than that in the white matter. The distribution of blood flow in brain tissues varies, and the blood flow in the gray matter (the part where brain cells are concentrated) is higher than that in the white matter (the part where nerve fibers are concentrated). The blood supply to the cerebral cortex is the richest, and ischemia in this part is prone to hemorrhagic cerebral infarction, and ischemia in the white matter is prone to ischemic infarction. The sensitivity of brain tissue to ischemic and hypoxic damage differs in different parts of the brain. Cortical and hippocampal neurons are sensitive to ischemic and hypoxic damage, so pathological damage of different degrees can occur in different parts of the brain. Blood circulation of the brain The brain tissue has almost no energy reserves and needs a continuous supply of oxygen and glucose from the blood circulation. Once the blood supply to the brain is impaired, the consequences are serious. (A) Blood supply to the brain The blood supply to the brain comes from the internal carotid artery system and the vertebrobasilar artery system, and the cerebral arteries branch repeatedly in the brain parenchyma up to the capillaries, and then gradually converge into veins. The deep and superficial veins of the brain first return to the dural sinus and then return to the heart via the internal jugular vein and other veins. Bounded by the parieto-occipital sulcus, the internal carotid artery system supplies blood to the anterior 3/5 of the cerebral hemisphere and part of the mesencephalon through the internal carotid artery, the anterior cerebral artery and the middle cerebral artery, while the vertebrobasilar artery system supplies blood to the posterior 2/5 of the cerebral hemisphere (the base of the occipital and temporal lobes) and part of the mesencephalon, brainstem and cerebellum. Both the internal carotid artery and the vertebrobasilar artery are located on the ventral side of the brain, so that the arterial branches of the brain emanate from the ventral side and then wrap around to the dorsal side of the brain, sending branches along the way to supply the various structures of the brain. (B) Rich collateral circulation 1. Collateral circulation of the extracranial arteries Anastomosis between the ophthalmic artery of the internal carotid artery and the facial artery and superficial temporal artery of the external carotid artery; anastomosis between the middle meningeal artery of the external carotid artery and the soft meningeal artery of the anterior, middle and posterior cerebral arteries; anastomosis between the external carotid artery and the vertebral artery through the occipital artery; extensive anastomosis between the branches of one or both external carotid arteries, and when one side of the common carotid artery When one side of the common carotid artery is occluded, the blood flow can flow from the healthy side to the affected side of the external carotid artery through these anastomoses and then from the lateral branches of the external carotid artery to the internal carotid artery to ensure the blood supply to the brain on the side of the lesion, which also plays a certain regulatory role when the cerebral blood supply is impaired. (1) The branches of the internal carotid artery and the basilar artery form an anastomosis at the base of the brain: it is called the ring of arteries at the base of the brain (Willis ring), that is, the anterior cerebral arteries on both sides are combined with each other by the anterior communicating arteries, and the internal carotid arteries on both sides and the posterior cerebral arteries on both sides are connected with each other by the posterior communicating arteries. The existence of the ring of the basilar artery plays an important role in the regulation and compensation of cerebral blood supply, and establishes a collateral circulation between the two cerebral arteries and between the internal carotid artery system and the vertebrobasilar artery system, which can compensate for the blood supply in case of stenosis or occlusion of one or more arteries in the neck. According to statistics, about 48% of the cerebral arterial rings in China are underdeveloped or abnormal, the more common ones are: the diameter of one posterior communicating artery is less than 1mm in about 27%; the posterior cerebral artery originates from the internal carotid artery in about 14%; the diameter of the anterior communicating artery is less than 1mm or missing; both anterior cerebral arteries originate from one internal carotid artery. Abnormal arterial rings are prone to aneurysm, and the junction of anterior communicating artery and anterior cerebral artery is a good site for aneurysm. (2) Anastomosis of each soft meningeal artery: the soft meningeal branches of the anterior, middle and posterior cerebral arteries form an anastomosis between the surfaces of the cerebral hemispheres, forming a rich collateral circulation, which can play a compensatory role to some extent if the branches of one cerebral artery become occluded. (3) The arteries supplying the cerebral hemispheres can be divided into cortical branches (or spiral branches) and central branches: cortical branches enter the soft membranes and anastomose into a network first, and then send out small branches from the anastomosing network to enter the cortex in a vertical direction to supply the cerebral hemispheres with cortical and subcortical white matter and the lateral side of the brainstem; central branches start from the proximal end of the arterial trunk, and they penetrate almost vertically into the brain parenchyma to supply They penetrate almost vertically into the brain parenchyma to supply the basal nucleus, thalamus, internal capsule, external capsule, etc. In the past, it was generally believed that the cortical and central branches penetrated into the brain parenchyma as noncoincident terminal arteries. In contrast, many experiments have now demonstrated the existence of pre-capillary anastomoses, but the anastomosing branches are so small that when a major vessel is obstructed or the brain is ischemic, it is difficult for such anastomoses to accomplish effective collateral circulation to maintain adequate amounts of blood flow, thus producing a focal point of ischemic softening in the distribution area of that artery. Cerebral arteries establish abundant collateral circulation through several groups of anastomotic branches, of which the Willis loop is the most important. Compared to the vascular system of other organs, the cerebrovascular system has two distinctive features: on the one hand, it has developed a very effective blood supply and compensatory guarantee mechanism through long-term evolution, and can be completely asymptomatic when one side of the internal carotid artery system or the middle cerebral artery is completely occluded; on the other hand, due to the presence of congenital variants or dysplasia of cerebrovascular disease, the possibility and the degree of effectiveness of the opening of the collateral circulation varies from person to person. The effectiveness of the opening of the collateral circulation is determined by the rate of occlusion of the main vessel. The slower the progression from stenosis to occlusion, the more complete the compensatory function of the collateral circulation, or even completely compensated without any clinical symptoms. In different patients, occlusion of the same artery can cause different symptoms, so it is difficult to determine the diseased vessel by clinical manifestations alone.