The veins of the thorax largely derive from the anterior cardinal veins, draining the region of the head and neck, and the posterior cardinal veins, draining the rest of the body. Both anterior and posterior veins join to form the common cardinal veins which return blood to the sinus venosus of the heart.
The right anterior cardinal vein and the right common cardinal vein derive the superior vena cava up to the point of junction with the azygous vein. Also, the former forms the right brachiocephalic vein. The left brachiocephalic vein is formed by an anastomotic vessel between the right and left anterior cardinal veins. The left posterior cardinal vein regresses except for vessels which drain the left second and third intercostal spaces - the future left superior intercostal vein.
From the fifth week of gestation, drainage of the thoracic wall is achieved by the newly-developed supracardinal veins which assume the site of the future azygous and hemiazygous veins. They take over the role of the posterior cardinal veins which regress. A small section of the posterior cardinal vein on the left remains to form the proximal part of the azygous vein.
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