Veins Spider Anatomy
Veins are thin-walled vessels that carry blood from the tissues. Veins normally permit the flow of blood in one direction: back toward the heart. Tiny valves inside the veins prevent blood flow from reversing direction, which is especially important in the legs. Leg muscle contractions squeeze blood out of veins, which helps to pump blood back toward the heart.
There are many deep and superficial (closer to the skin) veins in the body. The saphenous vein in the leg is one of the largest superficial veins.
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