Dorsal intercostal veins

The dorsal intercostal veins are paired vessels situated in each intercostal space along the dorsal thoracic wall that drain deoxygenated blood from the vertebrae, spinal cord structures, and intercostal muscles; they course cranially within the costal grooves alongside the intercostal arteries and nerves and empty into the azygos system (right or left depending on species), while also receiving tributaries (branches) from the dorsal muscular branches, spinal branches (via intervertebral veins), and collateral intercostal veins, thereby forming an important pathway for venous return from the dorsal thoracic wall and vertebral canal.