THE VENA CAVA. 667 



ward, terminates in the vena azygos major. Occasionally it opeas into the right 

 innominate vein. 



The Ifft x)if /("/ int- r<-"*t<il 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 

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

 spaces below the first, and. passing across the arch of the aorta, terminates in the 

 left innominate-vein. The left bronchial vein and the left superior phrenic open 

 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 

 innominate veins. It commences immediately below the cartilage of the first rib 

 close to the sternum on the right side, and, descending vertically, enters the peri- 

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

 of the right auricle opposite the upper border of the third left costal cartilage. In 

 its course it describes a slight curve, the convexity of which is turned to the right 

 side. 



Relations. In front, with the pericardium and process of cervical fascia which 

 i- continuous with it: this separates it from the thymus gland and from the ster- 

 num : 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 commencement of the innominate 

 artery and ascending part of the aorta. The portion contained within the peri- 

 cardium 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 several small veins from the pericardium and parts in the mediastinum. The 

 superior vena cava has no valves. 



The Azygos Veins connect together the superior and inferior venae cava, taking 

 the place of those vessels in thaf part of the chest occupied by the heart. 



The larger, or right azygos vein (vena azygos major), commences opposite the 

 first or second lumbar vertebra by a branch from the right lumbar veins (the 

 /'/>// lumbar); sometimes by a branch from the right renal vein or from the 

 inferior vena cava. It enters the thorax through the aortic opening in the Dia- 

 phragm, and passes along the right side of the vertebral column to the fourth 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 

 ] uissing 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, and is partly covered by 

 pleura. 



Tributaries. It receives, excepting those of the first three spaces, the intercostal 

 veins of the right side, the vena azygos minor, the left upper azygos vein, several 

 cesophageal. rnediastinal, and pericardial veins ; near its termination, the right 

 bronchial vein : and generally the right superior intercostal vein. A few imperfect 

 valves are found in this vein, but its tributaries are provided with complete valves. 



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

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



The l'-ff !<,>r>-r. or *>/><tHi / <i-'/<:/o$ vein (vena azygos minor), commences in the 

 lumbar region by a branch from one of the lumbar veins (ascending lumbar) or 

 from the left renal. It passes into the thorax through the left cms of the Dia- 

 phragm, and. ascending on the left side of the spine as high as the seventh or 

 eighth 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 inter- 

 C'>stal veins of the left side, and some oesophageal and mediastinal veins. 



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

 intercostal, with which it communicates above. It receives veins from 

 the intercostal spaces between the left superior intercostal vein and highest 

 tributary of the left lower azygos. They are usually two or three in number, 



