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Laboratory 21. Mediastinum
Step 12. The Azygos Vein and Posterior Intercostal Veins

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On the left side of the body, there is considerable variation in the pattern of venous drainage from the intercostal spaces posteriorly. Blood from the lower left intercostal spaces drains into the hemiazygos vein, which is a continuation of the left ascending lumbar vein. At about the level of the ninth thoracic vertebra, it turns and courses towards the right and becomes continuous with the azygos vein. More superiorly, the mid portion of the posterior intercostal spaces are drained by an accessory hemiazygos vein, which also crosses the midline and becomes continuous with the azygos vein around the level of the seventh or eighth thoracic vertebra.

Links and References:
Grant's: 1.86, 1.87
Netter: 226
Rohen/Yokochi: 261