608 THE VEINS 



the lower thoracic vertebrae as high as the eighth, where it turns obU(}uely to the 

 right, and, crossing in front of the spinal column behind the aorta and the a'sopha- 

 gus, opens into the vena azygos major. In its course it crosses over three or four 

 of the lower left intercostal arteries, and is covered by the pleura. 



Tributaries. — (1) The lower four left intercostal veins; (2) the left subcostal 

 vein; (o) the lower end of the third azygos vein (sometimes); (4) small left medi- 

 astinal veins; and (5) the lower left oesophageal veins. 



The vena azygos tertia (also called the upper left azygos and vena hemi- 

 azygos accessoriaj varies considerably in size, position, and arrangement, and is 

 often absent. It lies in the posterior mediastinum by the left side of the bodies of 

 the fifth, sixth, and seventh thoracic vertebrae, and is more or less vertical in 

 direction. It is continued above into the lower left superior intercostal vein, an<l 

 below either joins the vena azygos minor, or passes obliquely across the sixth or 

 seventh thoracic vertebra to join the vena azygos major. ^ The intercostal veins 

 intervening between it and the vena azygos minor then open directly across the 

 spine into the vena azygos major. It crosses the corresponding left intercostal 

 arteries, and is covered by the pleura. 



Tributaries. — (1) The fifth, sixth, and sometimes the seventh intercostal 

 veins; (2) the lower end of the lower left superior intercostal vein; (3) the upper 

 end of the vena azygos minor (sometimes); and (4) the left bronchial vein. 



The intercostal veins. — The intercostal veins are eleven in number on each 

 side, and correspond with the intercostal artei'ies. The last thoracic vein is here 

 called the subcostal, and is described separately (page 609). There is one vein 

 to each intercostal artery, the vein lying above the artery whilst in the intercostal 

 space. Each vein is joined by a dorsal tributary which runs with the dorsal l)ranch 

 of the intercostal artery between the transverse process of the vertebrae and the neck 

 of the rib. The dorsal tributaries return the blood from the muscles of the back, 

 and receive communicating branches from the dorsal spinal plexus and from the 

 spinal veins through the intervertebral foramina. The intercostal veins also receive 

 small tributaries from the bodies of the vertebrae. The termination of the inter- 

 (;ostal veins is different on the two sides, and is seldom alike in any two consecu- 

 tive subjects. 



On the right side. — The first intercostal vein (the upper right superior 

 intercostal vein) ascends with the superior intercostal artery, a branch of tlic 

 sul)clavian, to end either in the vertebral vein just before the latter joins the right 

 innominate vein, or in the right innominate direct. The second intercostal vein 

 either joins with the first, and opens with it as a common trunk into the vertebral 

 or innominate vein, or it joins with the third or with the third and fourth to open 

 into the vena azygos major as the latter is arching over the root of the right lung. 

 This vein is known as the lower right superior intercostal vein. The fifth, 

 sixth, seventh, eighth, ninth, tenth, and eleventh right intercostal veins join the 

 vena azygos major. The vipper of these have well-marked valves Avhere they join 

 the azygos vein. In the lower veins these valves are imperfect. All the intercostal 

 veins are provided with valves in their course between the muscles. 



On the left side the first intercostal vein also follows the superior intercostal 

 artery from the subclavian, and ascends to join the left vertebral or left innominate 

 vein, and is known as the upper left superior intercostal vein. The second 

 intercostal vein either joins the first, and opens with it as a common trunk into the 

 left vertebral or left innominate vein, or joins the third and fourth, as described 

 below, to form the lower left superior intercostal vein. The third and fourth, 

 and sometimes the second, intercostal veins unite to form a single trunk, the lower 

 left superior intercostal vein, which passes iipwards across the arch of the aorta, 

 and opens into the left innominate vein. This vein usually communicates at its 

 lower end with the third azygos vein, but at times crosses the spine, and enters 

 directly the vena azygos major. A fibrous cord can fretjuently be traced from 

 it through the vestigial fold of the pericardium to the oblicjue vein of ISIarshall 

 (page 605). The fifth and sixth, and sometimes tlie seventh, intercostal veins 

 cither end in the third azygos vein, or, if this is absent, cross the spine, and open 

 directly into the vena azygos major. The eighth, ninth, tenth, and eleventh, and 

 sometimes the seventh, intercostal veins join the vena azygos minor. 



