490 THE AZYGOS VEINS. 



are placed on the sides of the spine. In the lower part of the thorax 



Fig. 313. 





phagus, it passes in front of 



Fig. 313.— Sketch op the Principal Systemic 

 Venous Trunks, the Azrcos, and Intercostal 

 Veins. 



For the detailed description of this figure see 

 p. 473. Tlie following indications relate to the 

 accompanying part of the text:— S', the left superior 

 intercostal veins (the number is omitted on the 

 right side) ; 9, the main trunk of tJie azygos vein ; the 

 uppermost number marks its junction with the supe- 

 rior cava, the lowest its passage into the abdomen ; 9', 

 the left or hemiazygos ; 10, thoracic duct ; 11, inferior 

 vena cava ; 12, the union of a branch of the left 

 azygos with the left renal vein ; 13, 13', the right and 

 left azygos veins continued down into the abdomen, 

 and joining some of the lumbar veins; 15, union of 

 lumbar, ilio-lumbar, and sacral veins. 



the two veins of opposite sides are dis 

 posed symmetrically, but higher up the 

 blood gathered fi'om some of the veins of 

 the left side is poured into tlie trunk on 

 the right, which becomes enlarged and 

 unsymmetrical, and has on that account 

 received the name of main or riijht azyyos, 

 while the united lower veins from the 

 corresponding parts on tiie left side con- 

 stitute the small or left azijfjos. 



The right azygos vein (vena azygos 

 major) generally commences below by 

 branches from the lumbar veins (ascend- 

 ing lumbar) of the right side, and some- 

 times from the renal vein ; but not un- 

 frequently it receives a small branch from 

 the inferior cava, where that vessel turns 

 forwards to reach its opening in the dia- 

 phragm. Passing from the abdomen into 

 the thorax through the aortic opening in 

 the diaphragm, or to the outer side of 

 that opening through the fibres of the 

 right crus, the azygos vein ascends on the 

 bodies of the dorsal vertebrae, until it 

 arrives opposite the root of the right 

 lung, over which it arches forwards, and 

 then opens into the upper vena cava, im- 

 mediately above the point at which that 

 vessel is invested by the pericardium. 

 When passing through the opening in 

 the diaphragm, this vein is accompanied 

 by the thoracic duct, both being situated 

 on the right side of the aorta. In the 

 thorax, maintaining the same position 

 with respect to the duct and the oeso- 

 tho intercostal arteries, and is covered by 



