The superior vena cava, which returns to the heart all the venous blood from the upper part of the body, is formed by the union of the two innominate veins behind the first costo-sternal junction of the right side ; it proceeds down and opens into the upper part of the right auricle at the upper border of the third cartilage of the right side ; it is 3 inches long.

Fig. 75.—Venous Trunks,

1, Superior vena cava ; 2, subclavian vein ; 3, internal jugular ; 4, external jugular ; 5, azygos major ; 6, azygos minor superior ; 7, azygos minor inferior ; 8, thoracic duct; g, inferior vena cava ; 10, ascending lumbar vein ; 11, common iliac ; 12, external iliac; 13, internal iliac.

Its tributaries are the azygos major and small pericardiac and mediastinal veins.

The blood from most of the intercostal veins is received by the azygos veins.

The azygos major arises in the abdomen from the ascending lumbar vein on the right side, enters the thorax with the aorta, extends upwards on the bodies of the dorsal vertebrae, and joins the superior vena cava, thus connecting the two venae cavae.

The azygos minor, superior, joins the azygos major at the eighth intercostal space on the left side and communicates above with the left superior intercostal, which brings the blood from the second and third intercostal veins to the left innominate vein.

The azygos minor, inferior, arises from the left ascending lumbar vein, and joins the azygos major at the ninth intercostal on the left side.

The inferior vena cava, on entering the thorax, immediately passes through the pericardium and opens into the lower and back part of the heart.