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ANATOMY AND PATHOLOGY 
Septal branch 
L. coronary a. 
Pulmonary 
trunk 
Ventral (ant.) 
interventricular br. 
Great cardiac v. 
R. ventricle 
Tat ^AfiAov^ 
Figure 14.— Left coronary artery (dog) showing placement of an electromagnetic blood flow probe (BioTronex) 
around the origin of the artery. 
When the left azygos vein reaches the level 
of the fifth to sixth thoracic vertebra (a po- 
sition cranial to the hilus of the lung), the 
vein turns ventrally and empties into the cor- 
onary sinus (Figure 24). As the left azygos 
ascends on the dorsal thoracic v^^all, it is located 
on the left side of the descending thoracic aorta. 
The right azygos vein (formerly the azygos 
vein) is poorly developed in sheep, calf and pig 
and arises by small dorsal intercostal tributaries 
at the fifth and sixth thoracic vertebrae and 
continues cranially receiving dorsal intercostal 
veins at each intercostal space. After receiving 
its last dorsal intercostal vein it turns sharply 
dovi^nward and empties into the cranial vena 
cava at the level of the second thoracic vertebra 
(Figure 25). 
In man the hemiazygos, accessory hemiazygos 
and right superior intercostal veins empty into 
the azygos vein, vi^hile the left superior inter- 
costal, although communicating v^^ith the acces- 
sory hemiazygos vein, commonly drains into the 
left brachiocephalic vein (Figure 26). 
The Clinical Significance of Azygos Systems 
The azygos systems drain the thoracic wall 
and, by their connections to the lumbar veins, 
assist in the drainage of the abdominal wall. In 
the event of thrombosis or ligation of the caudal 
(inferior) vena cava below the level of the 
