470 VEINS. 



front of the trachea, behind the Sterno-thyroid muscles, communicating with ita 

 fellow by transverse branches, and terminates in the left vena innominata. The 

 right one, which is placed a little to the right of the median line, opens into the 

 right vena innominata, just at its junction with the superior cava. These veins 

 receive tracheal and inferior laryngeal branches, and are provided with valves at 

 their termination in the innominate veins. 



The Superior Intercostal Veins return the blood from the upper intercostal 

 spaces. 



The rigid superior intercostal, much smaller than the left, closely corresponds 

 with the superior intercostal artery, receiving the blood from the first, or first and 

 second intercostal spaces, and terminates in the right vena innominata. Some- 

 times it passes down, and opens into the vena azygos major. 



The left superior intercostal is always larger than the right, but varies in size in 

 different subjects, being small when the left upper azygos vein is large, and vice 

 versa. It is usually formed by branches from the two or three upper intercostal 

 spaces, and, passing across the arch of the aorta, terminates in the left vena inno- 

 minata. The left bronchial vein opens into it. 



The SUPERIOR VENA CAVA receives the blood which is conveyed to the heart 

 from the whole of the upper half of the body. It is a short trunk, varying 

 from two inches and a half to three inches in length, formed by the junction of 

 the two vena3 innominate. It commences immediately below the cartilage of the 

 first rib on the right side, and, descending vertically downwards, enters the peri- 

 cardium about an inch and a half above the heart, and terminates in the upper 

 part of the right auricle. In its course, it describes a slight curve, the convexity of 

 which is turned to the right side. 



Relations. In front, with the thoracic fascia, which separates it from the thymus 

 gland, and from the sternum ; behind, with the root of the right lung. On its 

 right side, with the phrenic nerve and the pleura of the right side ; on its left 

 side, with the ascending part of the aorta. The portion contained within the 

 pericardium is covered by the serous layer of that membrane, in its anterior 

 three-fourths. It receives the vena azygos major, just before it enters the peri- 

 cardium, and several small veins from the pericardium and parts in the medias- 

 tinum. The superior vena cava has no valves. 



The AZYGOS VEINS connect together the superior and inferior vense cavse, 

 supplying the place of these vessels in that part of the trunk in which they are 

 deficient, on account of their connection with the heart. 



The larger, the right azygos vein, commences opposite the first or second lumbar 

 vertebra, by receiving a branch from the right lumbar veins ; sometimes by a 

 branch from the renal vein, or from the inferior vena cava. It enters the thorax 

 through the aortic opening in the Diaphragm, and passes along the right side of 

 the vertebral column to the third dorsal vertebra, where it arches forward, over 

 the root of the right lung, and terminates in the superior vena cava, just before 

 that vessel enters the pericardium. Whilst passing through the aortic opening 

 of the Diaphragm, it lies with the thoracic duct on the right side of the aorta ; 

 and in the thorax, it lies upon the intercostal arteries, on the right side of the 

 aorta and thoracic duct, covered by the pleura. 



Branches. It receives nine or ten lower intercostal veins of the right side, the 

 vena azygos minor, several oesophageal, mediastinal, and vertebral veins ; near its 

 termination, the right bronchial vein ; and it is occasionally connected with the 

 right superior intercostal vein. A few imperfect valves are found in this vein ; 

 but its branches are provided with complete valves. 



The intercostal veins on the left side, below the two or three upper intercostal 

 spaces, usually form two trunks, named the left lower, and the left upper, azygos 

 veins. 



The left lower or smaller azygos vein commences in the lumbar region, by a 

 branch from one of the lumbar veins, or from the left renal. It passes into the 

 thorax, through the left crus of the Diaphragm, and, ascending on the left side of 



