It is formed by the folding of the serous layer over the remnant of the lower part of the left superior vena cava ( duct of Cuvier ), which becomes obliterated during fetal life, and remains as a fibrous band stretching from the highest left intercostal vein to the left atrium, where it is continuous with a small cardiac vein, the vein of the left atrium ( oblique vein of Marshall ), which opens into the coronary sinus.