Between the left pulmonary artery

Pulmonary artery

The pulmonary arteries carry deoxygenated blood from the heart to the lungs. They are the only arteries that carry deoxygenated blood....

 and subjacent pulmonary vein

Pulmonary vein

The pulmonary veins are large blood vessels that carry blood from the lungs to the left atrium of the heart. In humans there are four pulmonary veins, two from each lung...

 is a triangular fold of the serous pericardium

Pericardium

The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.-Layers:...

; it is known as the ligament of the left vena cava (vestigial fold of Marshall).

It is formed by the duplicature 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

Left atrium

The left atrium is one of the four chambers in the human heart. It receives oxygenated blood from the pulmonary veins, and pumps it into the left ventricle, via the mitral valve.-Foramen ovale:...

, where it is continuous with a small vein, the vein of the left atrium (oblique vein of Marshall), which opens into the coronary sinus

Coronary sinus

The coronary sinus is a collection of veins joined together to form a large vessel that collects blood from the myocardium of the heart. It is present in all mammals, including humans...

.