4.4.2. Arteries That Travel Inferiorly: Internal Thoracic Arteries
The two internal thoracic arteries branch off the two subclavian arteries, one on each side. Each internal thoracic artery runs down in a position that is posterolateral to the sternum but always posterior to the costal cartilages. Each artery extends to the tip of the costal margin (the costal margin is the inverted V-shape formed by the fused costal cartilages of ribs 7 -10. Throughout its course, each internal thoracic artery gives off anterior intercostal arteries that run in the costal groove of the corresponding rib. In the diagram, the anterior intercostal arteries are only shown on the left, but in life they are present bilaterally.
The anterior intercostal arteries anastomose with the posterior intercostal arteries (from the descending aorta). Essentially, the arteries fuse to allow the blood to reach the full length of the intercostal space.
At the tip of the costal margin, each internal thoracic artery then divides into two smaller branches: the superior epigastric artery and the musculophrenic artery.
The superior epigastric artery continues in the same direction as the internal thoracic artery. It will supply the rectus abdominis muscle. The superior epigastric artery will also anastomose with the inferior epigastric artery, which is an artery of the abdomen. The musculophrenic artery runs just posterior to the costal margin. It supplies the anterior portion of the lower intercostal spaces.