In this study, the macroanatomy of the azygos vein in the dog and sheep were compared to those of human. The obtained results showed that in dog it originates from cranial vena cava on the right side of thoracic cavity and the segmental veins arise from that as dorsal intercostal veins for draining blood from the vertebral column. In sheep the azygos vein lies on the left side of the thoracic cavity and originates from the coronary sinus of the right atrium. In human specimens, the azygos vein arise opposite the vertebra LI or LII at the junction between the right ascending lumbar vein and the right subcostal vein. The hemiazygos vein form on the left side from the lower three posterior intercostal veins, then ascends anterior of the vertebral column to the eighth thoracic level then cross the vertebral column posterior to the aorta, esophagus and thoracic duct and ends in the azygos vein. It concluded that the most anatomical similarities were seen between the azygos vein of the dog and human. However, the most anatomical differences were existed between the azygos vein of the sheep and human.
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