606 THE VEINS 



(b) A vein may run from the left innominate, or left superior intercostal vein, throu^^h the 

 vestigial fold of the pericardium to the coronary sinus, the left innominate vein being itself nor- 

 mal. This abnormality is similar in kind, but minor in degree, to the former, described under 

 (a). 



(2) Variations due to Persistence of the Left and Suppression of the Right Dad of Cuvier 



{a) The right innominate vein may cross the arch of the aorta to join a vertical left innom- 

 inate vein, and thus form a left superior vena cava, the normal right superior cava being 

 absent. The arrangement of the azj'gos veins under this condition n)ay be reversed, there being 

 a left vena azj'gos major opening over the root of the left lung into the left superior cava, and an 

 upper and lower right azygos vein arranged after the manner of the normal lei't azygos veins. 

 This arrangement of the veins may occur independently of any general transposition of the 

 viscera. 



(Jb) There are many other varieties, depending upon abnormalities in the normal development 

 of the great veins from the Cuvierian ducts and from the primitive jugular and cardinal veins ; 

 but these cannot be discussed here. 



THE AZYGOS VEINS 



The azygos veins (fig. 382) are three in number. They collect the blood 

 returned from the eleven intercostal spaces on the right side, and from the seven 

 or eight loAver intercostal spaces on the left side. They lie on each side of the 

 front of the bodies of the thoracic vertebrae, and establish a communication between 

 the superior and inferior venae cavae through the ascending lumbar veins. 



The vena azygos major l^egins in the abdomen, where it may be looked upon 

 as a continuation upwards of the ascending lumbar vein (page 633). It passes 

 through the aortic opening of the diaphragm, runs up through the posterior 

 mediastinum on the right side of the front of the bodies of the thoracic vertebrae as 

 high as the fourth thoracic vertebra; then curves forwards over the root of the right 

 lung, and opens into the superior vena cava immediately before the latter pierces 

 the pericardium. 



Relations. — (1) In the abdomen, the vena azygos major lies to the right of 

 the aorta and thoracic duct, under cover of the right crus of the diaphragm, upon 

 the first, sometimes upon the second, lumbar vertebra. At this spot it receives 

 the right subcostal vein, and is continued below into the ascending lumbar vein; 

 or, if the lower part of this vein is small, it ma}^ appear to begin in one of the 

 lumbar veins, or in the vena cava, or Avhere the portion of the ascending lumbar 

 vein which communicates with the renal is enlarged, in the renal vein. Tlu'ough 

 this intermediation of the ascending lumbar vein, a communication is established 

 between the iliac veins and the vena azygos. In obstruction of the inferior vena 

 cava much of the blood from the lower extremities and abdominal wall is 

 returned by the vena azygos major and its continuation, the ascending lumbar 

 vein, in the abdomen, to the vena cava superior. (See Ascending Lumbar 

 Vein, page 633. ) 



(2) At the aortic opening the vena azygos major lies to the right of the aorta 

 and thoracic duct, between the crura of the diaphragm. 



(3) In the posterior mediastinum, as it courses upwards on the right side of 

 the bodies of the thoracic vertebrae, the vena azygos major crosses in front of the 

 lower right intercostal arteries, having on its left side the descending aorta and 

 thoracic duct, in front the root of the right lung and lower down the pleura, and 

 on its right side the right ])leura and lung. 



(4) As it curls over the root of the right lung, the veua azygos major is in 

 contact with the right broiiclius, the pneumogastric nerve passing obliquely be- 

 tween them (fig. 324). 



It usually contains an imperfect pair of valves at the spot where it turns for- 

 wards from the fourth thoracic vertebra to arch over the root of the lung; and still 

 more imperfect valves are found at varying intervals lower down the vein. 



Tributaries. — (1) The vena azygos minor; (2) the vena hemiazygos accessoria, 

 or one or more of the left intercostal veins; (3) the lower end of the lower superior 

 intercostal vein of the left side (sometimes); (4) the lower right tcsophageal veins; 

 (a) a few right mediastinal veins; (6) the right broncliial vein; (7) the lower right 



