In the adult, the cordlike remnant of the fetal ductus arteriosus between the left pulmonary artery and the arch of the aorta. Just after birth, the ductus arteriosus constricts and begins to fill with endothelial cells, and within a few months the ductus arteriosus is completely closed. For the remainder of one’s life, the closed duct remains as a connecting cord attaching the two large outflow arteries of the heart.