SPINAL. 549 



inches in length, formed by the junction of the two vense innominate. It com- 

 mences immediately below the cartilage of the first rib on the right side, and, 

 descending vertically, enters the pericardium 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 thy- 

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

 its right side, with the phrenic nerve and right pleura ; 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 pericardium, and seve- 

 ral small veins from the pericardium and parts in the mediastinum. The supe- 

 rior vena cava has no valves. 



The Azygos Veins connect together the superior and inferior venas cavae, sup- 

 plying the place of those vessels in the part of the chest which is occupied by 

 the heart. 



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

 vertebra, by 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 ver- 

 tebral 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 cesophageal, mediastinal, and vertebral veins ; 

 near its termination, the right bronchial vein ; and 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 intercos- 

 tal 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 the spine, as high as the sixth or seventh dorsal vertebra, passes across the 

 column, behind the aorta and thoracic duct, to terminate in the right azygos 

 vein. It receives the four or five lower intercostal veins of the left side, and 

 some oasophageal and mediastinal veins. 



The left upper azygos vein varies according to the size of the left superior in- 

 tercostal. It receives veins from the intercostal spaces between the left superior 

 intercostal vein, and highest branch of the left lower azygos. They are usually 

 two or three in number, and join to form a trunk which ends in the right azy- 

 gos vein, or in the left lower azygos. When this vein is small, or altogether 

 wanting, the left superior intercostal vein will extend as low as the fifth or sixth 

 intercostal space. 



The bronchial veins return the blood from the substance of the lungs ; that of 

 the right side opens into the vena azygos major, near its termination ; that of 

 the left side, into the left superior intercostal vein. 



THE SPINAL VEINS. 



The numerous venous plexuses placed upon and within the spine may be 

 arranged into four sets. 



