There is one internal thoracic artery, previously called the internal mammary artery, originating from the first part of the subclavian artery, at the root of the neck on each side. It passes inferiorly, posterior to the respective brachiocephalic vein and medial to the scalenus anterior muscle. At this point, it is crossed by or may cross the phrenic nerve from its own side. The internal thoracic artery may supply an artery, the pericardiophrenic branch, that supplies the nerve and any pleura or pericardium that it may encounter. The internal thoracic nerve continues inferiorly deep to the clavicle, sternohyoid and sternothyroid muscles. Continuing horizontally downwards with the internal thoracic vein medially, it lies deep to its respective first costal cartilage and superficial to underlying parietal pleura. As it descends deep to and level with the costal cartilages, the internal thoracic artery gives off the:
- anterior intercostal arteries: two branches laterally to each of intercostal spaces one to six
- perforating arteries through the anterior part of the intercostal spaces; may have a role in supplying the breast, especially in females
Deep to the third intercostal space, the artery continues to run inferiorly but now superfical to the transversus thoracis muscle on the deep surface of the thoracic cage. At the level of the sixth intercostal space, the internal thoracic artery divides into the:
- superior epigastric arteries; head laterally to the margin of the xiphisternum and then inferiorly deep to the rectus sheath
- musculophrenic arteries; descend laterally in costodiaphragmatic recess
The internal thoracic artery is cloaked by venae comitantes which drain to the respective brachiocephalic vein via the internal thoracic vein.