a. Blood Supply of the Brain. The paired internal carotid arteries and the paired vertebral arteries supply blood rich in oxygen to the brain. Branches of these arteries join to form a circle under the base of the brain. This is called the cerebral circle (of Willis). From this circle, numerous branches supply specific areas of the brain.
(1) A single branch is often the only supply to that particular part of the brain. Such an artery is called an end artery. If it fails to supply blood to that specific area, the area will die (as in a stroke).
(2) The veins and venous sinuses of the brain drain into the paired internal jugular veins. These veins carry blood back toward the heart.
b. Blood Supply of the Spinal Cord. The blood supply of the spinal cord is by way of combination of three longitudinal arteries running along its length and reinforced by segmental arteries from the sides.
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