Anterior intercostal aa.

Twelve anterior intercostal aa. originate from the internal thoracic aa., one on the left side and one on the right side to each of the top six intercostal spaces. The lower five intercostal spaces are supplied anteriorly by anterior intercostal branches of the musculophrenic arteries.

In a given space, the anterior intercostal artery has a main upper branch that travels laterally along the bottom of the upper rib bounding the space until it anastomoses with its corresponding posterior intercostal artery (a branch of the thoracic aorta). Each anterior intercostal artery also gives off a smaller lower branch, which anastomoses with a collateral branch of the corresponding posterior intercostal artery.

The anterior intercostal aa. are at first situated between the pleura and the internal intercostal mm. More laterally, they lie between the internal intercostal mm. and the innermost intercostal mm.

The anterior intercostal aa. supply the intercostal muscles. Branches that perforate the external intercostal mm. also supply the pectoral muscles and the breasts.