This is the shriveled fibrous remnant of the ductus arteriosus of the fetus, a channel that short-circuited the lungs. It passes from the very commencement of the left pulmonary artery to the concavity of the aortic arch, beyond the point where the left subclavian artery branches off. It lies almost horizontally. The left recurrent laryngeal nerve hooks around it. The superficial part of the cardiac plexus lies anterior to it, and the deep part on its right, in front of the left main bronchus.
For the surgical approach to a patent ductus, the pleura over the aortic arch is incised behind the vagus nerve and upwards towards the origin of the left subclavian artery. The pleural flap is reflected forwards with the vagus and its left recurrent laryngeal branch to give sufficient access to the ductus.