THE AZYGOS SYSTEM. 895 



the hemiazygos. Occasionally, instead of opening into the superior vena cava, the azygos 

 terminates in the right subclavian, the right innominate vein, or even opens directly into the right 

 auricle. A further anomaly is sometimes presented by the azygos, in its upper part, being situated 

 at the bottom of a deep groove upon the surface of the right lung, which thus comes to have an 

 accessory lobe known as the azygos lobe or lobule. 



Practical Considerations. — The azygos veins are the connecting Hnks be- 

 tween the cardinal and the inferior caval systems. In cases of obstruction of the 

 inferior cava they are able to carry on the collateral circulation very effectively, 

 through their communication with the common iliac, renal, lumbar, and ilio-lumbar 

 veins. Growths in the posterior mediastinum, enlarged bronchial glands, or aortic 

 aneurisms may so compress these veins as to cause oedema of the chest wall by 

 interference with the intercostal veins which empty into them. 



The Hemiazygos Vein. 



The hemiazygos vein (v. hemiazygos) (Fig. 765), also called the azygos minor 

 inferior, is the counterpart, on the left side of the body, of the lower part of the azygos. 

 It arises just below the diaphragm as the continuation upward of the left ascending 

 lumbar vein (page 901), also receiving usually a communicating branch from the left 

 renal vein. It passes uDward into the thorax between the medial and intermediate 

 portions of the left crus of the diaphragm, and then ascends upon the left side of the 

 bodies of the lower .thoracic vertebrae, passing in front of the lower left intercostal 

 arteries and having the thoracic aorta to its right. At about the level of the eighth 

 or ninth vertebra it bends towards the right and, passing behind the aorta and the 

 oesophagus, opens into the azygos vein. 



In its course it receives the lower five or four left intercostal veins, which con- 

 stitute its principal tributaries, and in some cases it also receives the accessory hemi- 

 azygos vein. It also receives some branches from the oesophagus and from the 

 posterior mediastinum. 



The Accessory Hemiazygos Vein. 



The accessory hemiazygos vein (v, hemiazygos accessoria) (Fig. 765), also called 

 the azygos minor superior, is a descending stem which lies upon the left side of the 

 bodies of the upper thoracic vertebrae and receives the upper left intercostal \'eins. It 

 begins above at about the second intercostal space by the union of a small vein, which 

 connects it with the left innominate, with the left superior intercostal. When it has 

 reached the level of the seventh or eighth thoracic vertebra it bends to the right and, 

 passing beneath the aorta and the oesophagus, opens into the azygos vein. Quite 

 frequently it opens below into the hemiazygos just as that vein bends towards the 

 right, and e\'en when it has an independent connection with the azygos it may be 

 connected with the hemiazygos by an anastomotic branch. 



It receives the upper seven or eight left intercostal veins and in addition the left 

 posterior bronchial vein. 



Variations. — The hemiazygos and accessory hemiazygos veins together represent the left 

 cardinal vein of the embryo which primarily opened into the left Cuvierian dtict. With the dis- 

 appearance of the lower part of the left jugular vein the relations of the left cardinal change, 

 the vein making a connection across the middle line with the right cardinal (the azygos). Indi- 

 cations of the original condition are occasionally seen in a fibrous cord which connects the left 

 superior intercostal vein, which is strictly a portion of the accessory hemiazygos, with the 

 oblique vein of the left auricle (page 856). 



As already pointed out in speaking of variations of the azygos, cases have been observed 

 in which the hemiazygos and accessory hemiazygos occur upon the right side of the body, being 

 formed from the right cardinal, while the left cardinal gives rise to the azygos. And more 

 rarely the two veins have been observed fused to form a single trunk lying upon the anterior 

 surface of the thoracic vertebrae and receiving all the intercostal arteries. 



A considerable amount of variation exists in the number of intercostal veins received by the 

 hemiazygos and t^e accessory hemiazygos respectively. Usually they divide between them the 

 intercostals, since they either imite or cross the median line to the azygos over successive 

 vertebrae. The hemiazygos has been observed to cross the vertebral column anywhere from the 

 sixth to the eleventh vertebra, and the accessory may descend as far as the tenth or may cross at 

 the third. In cases where it makes its crossing high up a number of intercostal spaces may inter- 



