On each side, the superior intercostal artery supplies the posterior, superior two intercostal spaces.
Each arises from its ipsilateral costocervical trunk of the subclavian artery. It descends anterior to the neck of the first rib with sequentially on its medial side, the supreme intercostal vein and then the stellate ganglion and sympathetic trunk. Passing inferiorly and lateral to the vertebral bodies, it divides into its terminal branches at the level of the first interspace.
The third posterior intercostal artery anastomoses with the superior intercostal artery and indeed, the former may come to be the major arterial supply to the second intercostal space on each side.
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