The vessel in the fetus through which blood passes from the pulmonary artery to the aorta, thereby by-passing the lungs, which do not function during intra-uterine life. (See CIRCULATORY SYSTEM OF THE BLOOD.) The ductus normally ceases to function soon after birth and within a few weeks is converted into a fibrous cord (ligamentum arteriosum). Occasionally this obliteration does not occur: a condition known as patent ductus arteriosus (PDA). This is one of the more common congenital defects of the heart, and one which responds particularly well to surgical treatment. Closure of the duct can also be achieved in some cases by the administration of indomethacin. (See HEART, DISEASES OF.)