THE AZYGOS veins. 491 



the pleura. It is joined by the several veins which accompany the 

 aortic intercostal arteries of the right side ; and, at about the sixth 

 or seventh dorsal vertebra, by the left or smaller azygos vein. It is 

 also joined by several oesophageal and other small veins, and near its 

 termination by the bronchial vein of the right lung ; and it is generally 

 connected with the right superior intercostal vein. As it communicates 

 below with the vena cava inferior through one of the branches of that 

 large vein, while it terminates above in the vena cava superior, it forms 

 a connection between those two vessels. A few valves of imperfect 

 formation have been found in the azygos vein ; its branches (intercostal 

 veins) are provided with distinct valves. 



On the left side of the chest the veins of the three or four upper 

 intercostal spaces are usually united into one trunk, forming the lei't 

 superior intercostal vein, which (as already mentioned) is most fre- 

 quently united with the left innominate vein, but sometimes is connected 

 with the main azygos vein. 



Of the remaining left intercostal veins, one or two, generally about 

 the fifth and sixth, pass directly into the azygos ; while the lowest in 

 greater number unite almost constantly into one trunk, forming the 

 left or small azygos, which crosses to join the main azygos in the 

 neighbourhood of the seventh dorsal vertebra. There is frequently 

 union between these three sets of veins or their intercostal branches, 

 so that a part of one may be replaced by another, and the relative 

 size of the veins may be subject to considerable variation. 



The left lower or small azygos vein (vena hemiazygos) commences 

 from one of the lumbar veins (ascending lumbar), or from the left renal 

 vein, and, having entered the thorax with the aorta, or through the left 

 crus of the diaphragm, ascends upon the spine in front of the left inter- 

 costal arteries, receiving the lower intercostal veins of the left side ; 

 and passing behind the aorta, it opens into the right azygos vein, 

 opposite the sixth or seventh dorsal vertebra. 



Varieties. — The azygos vein lias been seen to receive the lower vena cava, 

 and, in such cases, is of coui-se extremely large. An example of this variety 

 exists in the Anatomical Musenm of Glasgow University. In one instance, 

 Meckel found the azygos ending in the subclavian vein. 



All the intercostal veins of the left side have been observed in some instances 

 to join a single vein, which ended in the left innominate ; the an-angement cor- 

 responding with that on the right side of the body. 



The bronchial veins return the blood employed in the nutrition of 

 the lungs. Their course corresponds with that of the bronchi, which 

 support them as they pass towards the root of the lungs. The bron- 

 chial vein of the right side opens into the trunk of the azygos vein 

 near its termination, that of the opposite side ends in the superior 

 intercostal vein. 



VEINS OF THE SPINE. 



The spinal veins form plexuses of closely anastomosing vessels along 

 the whole length of the spinal column. They have no valves. 



The veins within and upon the spinal column may be distinguished 

 into the following sets : a. The dorsal, placed deeply in the vertebral 

 grooves, and resting upon the spines and arches of the vertebrre : h. 

 The veins lodged within the bodies of the vertebrre : c. The anterior 

 longitudinal, two long series of veins, or rather venous plexuses, extend 



