abnormal vein morphology abnormal azygos vein morphology 2012-01-12T02:14:19Z MP:0011569 MPheno.ontology abnormal vena azygos morphology any structural anomaly of the unpaired vein which in humans arises from the right ascending lumbar vein or the vena cava, enters the thorax through the aortic orifice in the diaphragm, and terminates in the superior vena cava; unlike humans, mice have a single and left-sided azygos vein that develops from the paired embryonic cardinal venous system and drains most of the right and left thoracic walls into the left anterior vena cava csmith