=— eS Yy 
THE SUPERIOR VENA CAVA AND ITS TRIBUTARIES. 827 
commence in the substance of the walls of the heart, and terminate directly in its cavities, 
principally in the auricles ; some few, however, open into the ventricles. 
THE SUPERIOR VENA CAVA AND ITS TRIBUTARIES. 
The superior vena cava (Figs. 551 and 552) returns the blood from the head 
and neck, the upper extremities, the thoracic wall, and a portion of the upper part of 
the posterior wall of the abdomen. It is formed, immediately behind the lower part 
of the first right costal cartilage, by the union of the two innominate veins, and it 
descends, with a shght convexity to the right, to the level of the third right costal 
cartilage, where it opens into the upper and back part of the right auricle. It is 
about three inches (7-5 em.) long; in the lower half of its extent it is enclosed 
within the fibrous layer of the pericardium, and it is covered in front and later- 
ally by the serous layer. 
Relations.—It is overlapped in front by the margins of the right lung and pleural 
sac and by the ascending aorta. The lung and pleura intervene between it and the 
second and third costal cartilages, the internal intercostal muscles in the first and second 
intercostal spaces, and the internal mammary vessels. It is in relation behind with the 
right vagus nerve, the vena azygos major, the ‘right bronchus, the right pulmonary artery, 
and the upper right pulmon: wry vein. On its left side are the commencement of the 
innominate artery and the ascending portion of the aorta, whilst on the right side it is 
in close relation with the right pleura, the phrenic nerve and comes nervi phrenici 
vessels intervening. 
Tributaries.—In addition to the two innominate veins, by the union of which it is 
formed, the superior vena cava only receives one large tributary, viz. the vena azygos 
major ; but several small peric ardial and mediastinal veins open into it. 
THE AzyGcos VEINS. 
The vena azygos major (v. azygos, Fig. 592) commences either from the back of 
the inferior vena cava, at the level of the right renal vein, or as the direct upward 
continuation of an anastomosing channel which connects together the lumbar 
veins of the right side, and which is known as the right ascending lumbar vein. 
The great azygos vein ascends through the aortic orifice of the diaphragm, and 
is continued upwards through the posterior mediastinum. In the upper part of 
its course, it first. passes behind and then arches forwards above the root of the 
right lung to its termination in the posterior part of the superior vena cava, 
immediately before the latter vessel pierces the pericardium. It frequently 
possesses imperfect valves. 
Relations.—/n the abdomen it lies on the bodies of the upper lumbar vertebree, 
behind the right crus of the diaphragm and the inferior vena cava, and to the right side 
of the thoracic duct. 
In the thorax it lies on the bodies of the lower eight dorsal vertebrae, the intervening 
discs, and the anterior common ligament, and it crosses in front of the right aortic inter- 
costal arteries. In the lower part of the posterior mediastinum it is covered in front by the 
right pleura and lung ; at a higher level it is overlapped by the right margin of the ceso- 
phagus, and immediately before its termination it is crossed by the root of the right lung. 
On its right side it receives the right posterior intercostal veins. On its left side it is 
in relation, in the greater part of its extent, with the thoracic duct and, as it arches for- 
wards over the root of the lung, with the right vagus nerve. About the level of the seventh 
dorsal vertebra it receives the vena azygos minor superior, whilst at the level of the 
eighth dorsal vertebra the vena azygos minor inferior opens into it. 
In addition to the left azygos veins it receives the right posterior intercostal veins, 
except that from the first space but including the right superior intercostal vein, the 
right subcostal vein, and, through the ascending lumbar vein, the upper right Jambar 
veins. It also receives the right bronchial veins and some small cesophageal, pericardial, 
and mediastinal tributaries. 
The vena azygos minor superior (v. hemiazygos accessoria) is formed by the union of 
the fourth, fifth, sixth, and seventh left posterior intercostal veins. It lies in the posterior 
mediastinum on the left sides of the bodies of the fifth, sixth, and seventh dorsal vertebree, 
and crosses the spine from left to right opposite the body of the seventh dorsal vertebra, 
passing behind the aorta, cesophagus, and thoracic duct ; it terminates in the vena azygos 
ae 
