VERTEBRAL AND SPINAL VEINS. 343 



that part of the trunk 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 

 communication 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 upon 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 beneath the border of the diaphragm, and ascending upon 

 the left side of the 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 or six upper intercostal veins of the left side. It communi- 

 cates below with the vena azygos minor, and ascends to terminate in 

 the left vena innominata. 



VERTEBRAL AND SPINAL VEINS. 



The numerous venus 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, trans- 

 verse and articular processes and arches of the vertebra. They 

 receive the returning blood from the dorsal muscles and surrounding 

 structures, and transmit it, in part to the meningo-rachidian, and in 

 part to the vertebral, intercostal, lumbar, and sacral veins. 



The Meningo-rachidian veins are situated between the theca ver- 

 tebralis and the vertebrae. They communicate freely with each 

 other by means of a complicated plexus. In front they form two 

 longitudinal trunks, which extend the whole length of the column on 

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

 body of each vertebra by transverse trunks, which pass beneath the 

 ligament, and receive the large vertebral veins from the interior of 

 each vertebra. They pour their blood into the vertebral veins in 



