4.4.1. Arteries That Travel Inferiorly: Descending Aorta
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The descending aorta always runs in close proximity to the vertebral column - specifically, it runs on the anterolateral aspect of the vertebral column. In life, however, the two are not touching, as ligaments and fascia separate them.
The posterior intercostal arteries exit from the lateral aspect of the descending aorta and run with their associated ribs in their costal grooves (see Section 2.6.). You can see the posterior intercostal arteries, but not the ribs, in the accompanying diagram.
The 3rd - 11th posterior intercostal arteries are direct branches of the descending aorta. Remember that the first and second intercostal arteries are branches of the costocervical trunk.
The artery that runs in the costal groove of rib 12 is not called the twelfth intercostal artery (remember there is no twelfth intercostal space). The artery here is named the subcostal artery. Remember that there are only 11 intercostal muscles on each side, so the subcostal artery is actually going to travel inferiorly to supply the muscles of the abdominal wall.