Anterior intercostal arteries that supply the upper six intercostal spaces arise as lateral branches from the internal thoracic artery, whereas those supplying the lower spaces arise from the musculophrenic artery.
In each intercostal space, the anterior intercostal arteries usually have two branches:
One passes below the margin of the upper rib.
The other passes above the margin of the lower rib and meets a collateral branch of the posterior intercostal artery.