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.
Key words: Azygos vein, human, sheep, dog