348 VERTEBRAL AND SPINAL VEINS. 



AZYGOS VEINS. 



The azygos veins (fig. 165) form a system of communication between 

 the superior and inferior vena cava, and serve to return the blood from 

 that part of the trunk of the body in which those vessels are deficient, on 

 account of their connexion with the heart. This system consists of three 

 vessels, the 



Vena azygos major, 



Vena azygos minor, 



Superior intercostal vein. 



The Vena azygos major commences in the lumbar region by a commu- 

 nication with the lumbar veins ; sometimes it is joined by a branch directly 

 from the inferior vena cava, or by -one from the renal vein. It passes 

 through the aortic opening in the diaphragm, and ascends along the right 

 side of the vertebral column to the third dorsal vertebra, where it arches 

 forwards over the right bronchus, and terminates in the superior cava. It 

 receives all the intercostal veins of the right side, the vena azygos minor, 

 and the bronchial veins. 



The Vena azygos minor commences in the lumbar region, on the left 

 side, by a communication with the lumbar or renal veins. It passes be- 

 neath the border of the diaphragm, and, ascending along the left side of 

 die vertebral column, crosses the fifth or sixth dorsal vertebra to open into 

 the vena azygos major. It receives the six or seven lower intercostal 

 veins of the left side. The azygos veins have no valves. 



The Superior intercostal vein is the trunk formed by the union of the five 

 Ar six upper intercostal veins of the left side. It communicates below with 

 ihe vena azygos minor, and ascends to terminate in the left vena innominata. 



VERTEBRAL AND SPINAL VEINS. 



The numerous venous plexuses of the vertebral column and spinal cord 

 may be arranged into three groups : 



Dorsi-spinal, 



Meningo-rachidian, 



Medulli-spinal, 



The Dorsi-spinal veins form a plexus around the spinous, transverse and 

 xrticular processes, and arches of the vertebrae. They receive the return- 

 ng blood from the dorsal muscles and surrounding structures, and trans- 

 wit it, in part to the meningo-rachidian, and in part to the vertebral, in- 

 tercostal, lumbar, and sacral veins. 



The Meningo-racJddian veins are situated between the theca vertebralis 

 and the vertebrae. They communicate freely with each other by means 

 of a complicated plexus. In front they form two longitudinal trunks, 

 (longitudinal spinal sinuses ,) which extend the whole length of the column 

 on each side of the posterior common ligament, and are joined on the 

 bor'y of each vertebra by transverse trunks, which pass beneath the liga- 

 ment, and receive the large basi-vertebral veins from the interior of each 

 vertebra. The meningo-rachidian veins communicate superiorly through 

 the anterior condyloid foramina with the internal jugulars ; in the neck 

 they pour their blood into the vertebral veins; in the thorax, into the in- 



