There are eleven posterior intercostal arteries which supply the intercostal spaces from the dorsal aspect of the thoracic cage. They are derived from the:
- superior intercostal artery: supplies the most superior two interspaces
- thoracic aorta:
- supplies the remaining nine pairs of intercostal arteries and a pair of subcostal arteries
- branches pass over the vertebral body at each level and then anterior and laterally to each transverse process
Each posterior intercostal artery branches within each interspace:
- dorsal branch given off next to the vertebral body and passes posteriorly
- main branch passes obliquely to the angle of the superior rib within the interspace and then divides into:
- large superior branch which runs around the intercostal space within subcostal groove of the superior rib. Within this layer it lies superficial to the innermost intercostal muscles, inferior to the intercostal vein and superior to the intercostal nerve.
- small collateral posterior intercostal artery; passes around intercostal space superior to the inferior rib in the same anatomical plane.
Roughly half way around the interspace each posterior intercostal nerve:
- merges with smaller collateral anterior intercostal nerves; this does not occur in the most inferior two interspaces which have no supply from the anterior intercostal arteries
- gives off the lateral cutaneous branch of each interspace