Veins of the Thorax The brachiocephalic vein of the thorax carries blood that is on its way back from the head, neck and upper limbs, as well as collects blood from the mammary glands and from the first two or three intercostals spaces. The right and left brachiocephalic veins unite to form the superior vena cava. The left brachiocephalic vein is longer than the right because of the superior vena cava's location to the right of the body's midline. Most blood from the thoracic wall and tissues empties into an amalgamation of veins called the azygos system. This system offers a collateral pathway for draining areas served by the inferior vena cava, including the abdominal wall. A variety of anastomoses connect the azygos system with the inferior vena cava. The veins of the azygos system border the vertebral column laterally. Large veins draining the lower limbs and abdomen empty into the azygos system. The azygos system can drain venous blood from the lower body if the inferior vena cava or hepatic portal veins are damaged. The azygos vein arises from the right ascending lumbar vein that drains most of the right abdominal cavity wall and the right posterior intercostal veins that drain chest muscles. It empties into the superior vena cava. The hemiazygos vein arises from the left ascending lumbar vein and lower posterior intercostal veins, and it ends by merging with the azygos vein. It collects blood from the ninth through eleventh left posterior intercostal, esophageal, mediastinal, and occasionally the accessory hemiazygos veins and drains the left side of thoracic wall, thoracic viscera, and abdominal wall. Theaccessory hemiazygos vein collects blood from the fourth through eighth left posterior intercostal, left bronchial, and mediastinal veins. It ends by merging with the azygos vein. .