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The descending thoracic aorta has nine paired sets of posterior intercostal arteries. These supply intercostal spaces 3 to 11. There may be some anastomosis with the superior intercostal artery within the second intercostal space on each side.
The branches to the right intercostal spaces are longer than those on the left as they must traverse the anterior surfaces of the vertebral bodies, deep to the thoracic duct, oesophagus and azygous vein, from their aortic origin on the left side of the thoracic vertebral bodies. On the left side, the arteries pass laterally deep to the superior intercostal vein superiorly and the hemiazygous system inferiorly.
On both sides, the intercostal arteries run posterior to lung and costal pleura with the sympathetic trunks crossing them near the heads of the ribs. Further, the splanchnic nerves may also traverse the anterior surfaces of the lower intercostal arteries.
Each intercostal artery passes obliquely and laterally to the angle of the superior rib within each interspace where it locates into the subcostal groove. From here, it gives off a collateral that runs more inferiorly within the intercostal space. The superior seven pairs of arteries anastomose within the intercostal spaces with the anterior intercostal nerves. The most inferior two pairs of posterior intercostal arteries continue anteriorly into the anterior abdominal wall.
On route, each posterior intercostal artery gives off dorsal and lateral branches.
The posterior intercostal arteries are in series with the subcostal pair of arteries.
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