THE AZYGOS VEIN AND ITS TKIBUTAKIES. 961 



the right aortic intercostal arteries. In the lower part of the posterior mediastinum the 

 right pleura and lung lie anterior to it ; at a higher level it is overlapped by the right 

 margin of the oesophagus, and immediately before its termination it is crossed by the root 

 of the right lung. 



On its left side it is in relation, in the greater part of its extent, with the thoracic duct and, 

 as it arches anteriorly over the root of the lung, with the right vagus nerve and the trachea. 

 About the level of the eighth thoracic vertebra it receives the accessory hemiazygos vein, 

 whilst at the level of the ninth thoracic vertebra the hemiazygos vein opens into it. 



In addition to the two veins last mentioned it receives the right posterior intercostal 

 veins, except that from the first space but including the right superior intercostal vein, 

 the right subcostal vein, and, through the ascending lumbar vein, the upper right lumbar 

 veins. It also receives the right bronchial veins and some small oesophageal, pericardial, 

 and mediastinal tributaries. 



Tributaries. (1) The vena hemiazygos accessoria (O.T. vena azygos minor 

 superior) is formed by the union of the fourth, fifth, sixth, seventh and eighth left 

 posterior intercostal veins. It lies on the left sides of the bodies of the fifth, sixth, and 

 seventh thoracic vertebrae, and the corresponding intercostal arteries. It crosses the 

 vertebral column, from left to right, opposite the body of the eighth thoracic vertebra, 

 passing posterior to the aorta, oesophagus, and thoracic duct ; and it terminates either in 

 the vena azygos or in the vena hemiazygos. In addition to its intercostal tributaries 

 it receives the left bronchial veins, and some small posterior mediastinal veins, and it 

 communicates with the left superior intercostal vein. 



(2) The vena hemiazygos commences in the epigastric region of the abdomen. At its 

 origin it is connected either with the left ascending lumbar vein or with the left renal vein. 

 After piercing the left crus of the diaphragm it ascends, on the left sides of the bodies of the 

 lower thoracic vertebrae, and, opposite the eighth or ninth thoracic vertebra, it turns to 

 the right, crosses the front of the vertebral column, posterior to the aorta, oesophagus, and 

 thoracic duct, and terminates in the vena azygos. As it ascends, on the bodies of the 

 vertebrae, it lies lateral to the aorta, and medial to the roots, of the splanchnic 

 nerves, and anterior to the lower left intercostal arteries. Through the left ascending 

 lumbar vein it receives blood from the upper lumbar veins of the left side ; the left 

 subcostal vein, the lower three posterior intercostal veins, and small mediastinal tributaries 

 also terminate in it. 



(3) The bronchial veins do not quite correspond to the bronchial arteries, and they are 

 not found on the walls of the smallest bronchi. On each side the tributaries run, anterior 

 or posterior to the bronchial tubes to reach the root of the lung, where they unite, as a rule, 

 into two small trunks ; those of the right side open into the vena azygos, and those of the 

 left into the accessory hemiazygos vein, or into the left superior intercostal vein. On 

 both sides they are joined by tracheal and posterior mediastinal veins. Some few small 

 bronchial veins, including most of those from the smaller tubes, open into the pulmonary 

 veins. 



(4) Venae Intercostales. There are two sets of intercostal veins, the anterior 

 and the posterior. 



The anterior intercostal veins are tributaries of the internal mammary or of the 

 musculo-phrenic veins, and are described with those vessels (pp. 962, 963). 



The posterior intercostal veins (Fig. 798) are eleven in number on each side. A 

 single vein runs in each intercostal space it is situated in the costal, groove, above the 

 corresponding artery. 



On the right side the posterior intercostal vein of the first space accompanies the 

 superior intercostal artery across the front of the neck of the first rib, and terminates 

 in the vertebral or innominate vein. The second, third, and fourth posterior intercostal 

 veins of the right side unite together to form a common trunk, the right superior 

 intercostal vein, which terminates by joining the vena azygos. The fifth to the 

 eleventh posterior intercostal veins of the right side open separately into the vena azygos. 



On the left side the first posterior intercostal vein follows a course similar to that 

 taken by the corresponding vein on the right side, and terminates in the left vertebral or 

 innominate vein. The second, third, and sometimes the fourth posterior intercostal veins 

 of the left side unite to form the left superior intercostal vein, which runs from behind 

 forwards along the left or anterior aspect of the aortic arch. It passes obliquely between 

 the left vagus and phrenic nerves, crosses the root of the left subclavian artery, and ends 

 in the lower border of the left innominate vein. The fifth, sixth, seventh, and eighth, and 

 sometimes the fourth posterior intercostal veins of the left side terminate in the accessory 

 hemiazygos vein, and the ninth, tenth, and eleventh end in the hemiazygos vein. 



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