The subcostal arteries, so named because they lie below the last ribs, constitute the lowest pair of branches derived from the thoracic aorta, and are in series with the intercostal arteries.
Each intercostal artery is accompanied by the corresponding (i.e. ipsilateral) subcostal vein and nerve.
[2] Before entering the anterior abdominal wall, each runs laterally upon the anterior surface of the lumbar fascia (and thus also anterior to the underlying quadratus lumborum muscle which the lumbar fascia envelops) posterior to the ipsilateral kidney.
[2] It anastomoses with the superior epigastric, lower intercostal, and lumbar arteries.
[2] This article incorporates text in the public domain from page 601 of the 20th edition of Gray's Anatomy (1918)