THE VEINS OF THE VERTEBRAL COLUMN 667 



Relations. In front are the anterior margins of the right lung and pleura with the pericardium 

 intervening below; these separate it from the first and second intercostal spaces and from the 

 second and third right costal cartilages; behind it are the root of the right lung and the right 

 vagus nerve. On its right side are the phrenic nerve and right pleura; on its left side, the com- 

 mencement of the innominate artery and the ascending aorta, the latter overlapping it. Just 

 before it pierces the pericardium, it receives the azygos vein and several small veins from the 

 pericardium and other contents of the mediastinal cavity. The portion contained within the 

 pericardium is covered, in front and laterally, by the serous layer of the membrane. The superior 

 vena cava has no valves. 



The azygos vein (v. azygos; vena azygos major) begins opposite the first or second 

 lumbar vertebra, by a branch, the ascending lumbar vein (page 678) ; sometimes by 

 a branch from the right renal vein, or from the inferior vena cava. It enters the 

 thorax through the aortic hiatus in the diaphragm, and passes along the right side 

 of the vertebral column to the fourth thoracic vertebra, where it arches forward 

 over the root of the right lung, and ends in the superior vena cava, just before 

 that vessel pierces the pericardium. In the aortic hiatus, it lies with the thoracic 

 duct on the right side of the aorta ; in the thorax it lies upon the intercostal arteries, 

 on the right side of the aorta and thoracic duct, and is partly covered by pleura. 



Tributaries. It receives the right subcostal and intercostal veins, the upper three 

 or four of these latter opening by a common stem, the highest superior intercostal 

 vein. It receives the hemiazygos veins, several esophageal, mediastinal, and peri- 

 cardial veins, and, near its termination, the right bronchial vein. A few imperfect 

 valves are found in the azygos vein; but its tributaries are provided with complete 

 valves. 



The intercostal veins on the left side, below the upper three intercostal spaces, 

 usually form two trunks, named the hemiazygos and accessory hemiazygos veins. 



The Hemiazygos Vein (v. hemiazygos; vena azygos minor inferior) begins in the 

 left ascending lumbar or renal vein. It enters the thorax, through the left cms 

 of the diaphragm, and, ascending on the left side of the vertebral column, as high 

 as the ninth thoracic vertebra, passes across the column, behind the aorta, esoph- 

 agus, and thoracic duct, to end in the azygos vein. It receives the lower four 

 or five intercostal veins and the subcostal vein of the left side, and some esophageal 

 and mediastinal veins. 



The Accessory Hemiazygos Vein (v. hemiazygos accessorial vena azygos minor supe- 

 rior) descends on the left side of the vertebral column, and varies inversely in size 

 with the highest left intercostal vein. It receives veins from the three or four 

 intercostal spaces between the highest left intercostal vein and highest tributary 

 of the hemiazygos; the left bronchial vein sometimes opens into it. It either crosses 

 the body of the eighth thoracic vertebra to join the azygos vein or ends in the 

 hemiazygos. When this vein is small, or altogether wanting, the left highest 

 intercostal vein may extend as low as the fifth or sixth intercostal space. 



In obstruction of the superior vena cava, the azygos and hemiazygos veins are one of the 

 principal means by which the venous circulation is carried on, connecting as they do the superior 

 and inferior vense cavae, and communicating with the common iliac vein? by the ascending lumbar 

 veins and with many of the tributaries of the inferior vena cava. 



The Bronchial Veins (vv. bronchioles} return the blood from the larger bronchi, and 

 from the structures at the roots of the lungs; that of the right side opens into the 

 azygos vein, near its termination; that of the left side, into the highest left inter- 

 costal or the accessory hemiazygos vein. A considerable quantity of the blood which 

 is carried to the lungs through the bronchial arteries is returned to the left side of 

 the heart through the pulmonary veins. 



The Veins of the Vertebral Column (Figs. 578, 579). 



The veins which drain the blood from the vertebral column, the neighboring 

 muscles, and the meninges of the medulla spinalis form intricate plexuses extending 





