The internal mammary artery is a long artery that is one of three branches of the first part of the subclavian artery.

 

It arises from the lower surface of the subclavian near the medial border of the scalenus anteriorscalenus anterior, and runs downwards and medially, through only a small part of the neck, to enter the thorax behind the first costal cartilage.

 

In the neck, the internal mammary lies on the pleura behind the medial part of the clavicle. The left artery is behind the end of the subclavian vein and the beginning of the innominate artery. The right artery is behind the internal jugular vein and innominate vein; and the phrenic nerve crosses it from lateral to medial side and usually in front of it.

 

The artery is not accompanied by a vein in the neck, for the internal mammary vein ends in the innominate at the inlet of the thorax.