662 THE BLOOD-VASCULAR SYSTEM 



THE VEINS OF THE THORAX 

 THE SUPERFICIAL VEINS OF THE THORAX 



The superficial veins of the front of the thorax can be seen in fig. 537. They 

 form a plexus over the entire chest which the portion over the mammary gland is 

 called the mammary plexus. The laterally placed lateral thoracic and costo- 

 axillary veins drain the mammary plexus and communicate with the thoraco- 

 epigastric vein. These three veins terminate in the axillary vein (p. 671). The 

 veins nearer the median line are drained by the internal mammary vein and its 

 anterior intercostal and superior epigastric tributaries. The veins over the entire 

 thorax are in free communication with the superficial veins of the abdominal wall 

 (p. 683). 



THE DEEP VEINS OF THE THORAX 



The deep veins of the thorax are: — the pulmonary veins, and the vena cava 

 superior and its innominate and other tributaries. Of these veins, the pulmonary, 

 the vena cava superior, and the innominate veins have already been described, 

 as have the tributaries of the latter arising in the neck. 



The following veins are described below: — (1) The azygos and ascending 

 lumbar veins, which discharge their blood into the vena cava superior; (2) the 

 veins of the vertebral column, which are tributary to the azygos veins through the 

 intercostals; (3) the internal mammary veins, and (4) the superior phrenic, an- 

 terior mediastinal and pericardiac veins, all of which open into the innominate 

 veins. 



I. THE AZYGOS AND ASCENDING LUMBAR VEINS 



The azygos veins are longitudinal veins, the remnants of the posterior cardi- 

 nals, which are the main collecting trunks for the posterior part of the body in 

 the embryo. They lie along the sides of the thoracic vertebrae, and collect the 

 blood from the intercostal veins; they are the upward continuation of longitudinal 

 anastomotic trunks, the ascending lumbar veins which take origin in the abdomen. 

 The azygos veins are three in number, the azygos (azygos major) on the right side, 

 and the hemiazygos (azygos minor) and accessory hemiazygos (azygos tertia) on 

 the left. 



The azygos vein [v. azygos] begins in the abdomen as a continuation .upward of 

 the ascending lumbar vein. Through this means it connects with the iliac veins 

 and it has also an anastomosis with the vena cava inferior which may become very 

 important in cases of obstruction of the vena cava. It runs up through the pos- 

 terior mediastinum on the right side of the front of the bodies of the thoracic 

 vertebrae as high as the fourth thoracic vertebra, in this part of its course lying to 

 the right of the aorta and thoracic duct; it then curves forward over the root of the 

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

 pierces the pericardium. 



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

 ward 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. 



It receives the intercostal veins of the right side, except the first two or three. 

 These veins (usually excepting the first) are collected into a common trunk 

 before joining the azygos vein. It also receives the hemiazygos and accessory 

 hemiazygos, the right posterior bronchial vein, and small oesophageal and pos- 

 terior mcfliastinal veins. 



The hemiazygos vein [v. hemiazygos] begins in the abdomen by communicat- 

 ing, like the azygos vein, with the ascending lumbar vein of its own side. It 

 courses up the posterior mediastinum to the left of the bodies of the lower thoracic 

 vertebraj as higli as the eighth or ninth, wliore it turns obliquely to the right, and, 

 crossing in front of the vertebral column behind the aorta and the a\sophagus, 

 opens into the vena azygos. In its course it crosses over three or four of the lower 

 left intercostal arteries, and is covered by the pleura. 



