VERTEBRAL AND SPINAL VEINS. 383 



It passes through the aortic opening in the diaphragm, and ascends 

 upon the right side of the vertebral column to the third dorsal verte- 

 bra, 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 ver- 

 tebra 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 communicates 

 below with the 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 articular processes, and arches of the vertebrae. 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 verte- 

 bralis and the vertebrae. They communicate freely with each other 

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

 nal 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 body of each vertebra by transverse trunks, which pass 

 beneath the ligament, and receive the large basi-vertebral veins 

 from the interior of each vertebra. The meningo-rachidian veins com- 

 municate 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 intercostals ; and in the loins and pelvis 

 into the lumbar and sacral veins, the communications being established 

 through the intervertebral foramina. 



The Medulli-spinal veins are situated between the pia mater and 

 arachnoid ; they communicate freely with each other to form plexuses, 

 and send branches through the intervertebral foramina with each of the 

 spinal nerves, to join the veins of the trunk. 



