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.
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. It then pierces the posterior aponeurosis of the transversus abdominis, thus entering the anterior abdominal wall to course in between the abdominal internal oblique muscle and transverse abdominal muscle (the neurovascular plane of the anterior abdominal wall).
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