THE NERVOUS SYSTEM 1333 



the intercostal veins; and in the lumbar region by veins which 

 join the dorsal branches of the lumbar veins. 



The veins of the bodies of the vertebrae {vence basis vertehrce) are 

 contained within the cancellated tissue of the vertebral bodies. 

 They communicate in front with the anterior extraspinal veins, 

 and posteriorly they terminate in two venous trunks which, 

 emerging through the two foramina on the posterior surface of each 

 vertebral body, open into the transverse communicating branch 

 between the two anterior longitudinal intraspinal veins. 



The anterior longitudinal intraspinal veins form two anastomotic 

 chains, which are situated on the posterior surfaces of the bodies 

 of the vertebrae, one on either side. They communicate with each 

 other opposite the centre of each body by transverse branches 

 which receive the terminal trunks of the venae basis vertebrae. 

 These transverse branches pass between the posterior common 

 ligament and the bodies of the vertebrae. Superiorly the anterior 

 intraspinal veins communicate with the vertebral and the trans- 

 verse or basilar plexuses of veins, and laterally an offset passes 

 outwards through each intervertebral foramen, which, with that 

 of the posterior intraspinal vein, forms a plexus around the adjacent 

 spinal nerve. 



The posterior longitudinal intraspinal veins are situated in front 

 of the laminae, one on either side, and they are connected at 

 frequent intervals by transverse branches. They communicate 

 with the dorsal spinous venous plexus by branches which pierce 

 the ligamenta subflava. Superiorly they communicate with the 

 marginal sinuses, on either side of the foramen magniun and vermi- 

 form fossa, which by their union form the occipital sinus. With 

 the marginal sinuses and the anterior intraspinal veins they form a 

 venous ring at the foramen magnum. Laterally each vein sends 

 outwards through the corresponding intervertebral foramen an 

 offset, which, with that of the anterior intraspinal vein, forms a 

 plexus around the adjacent spinal nerve. 



The anterior and posterior intraspinal veins are situated between 

 the theca of dura mater and the wall of the spinal canal. 



The anterior extraspinal veins form a plexus along the anterior 

 aspect of the bodies of the vertebrae, which is most copious in the 

 neck. On either side it communicates with the vertebral plexus, 

 around the vertebral artery in the neck, the intercostal veins in the 

 thoracic region, and the lumbar veins in the lumbar region. It is 

 also connected with the venae basis vertebrae. 



Veins of the Spinal Cord. — ^These vessels lie within the substance 

 of the pia mater, and are disposed as venous chains, one being in 

 front, one behind, and two on either side. The anterior vessel lies 

 over the anterior median fissure beneath the anterior spinal artery ; 

 the posterior vessel is also m.esially placed ; and the two lateral 

 vessels are situated one in front of, and the other behind, the 

 posterior nerve-roots. Besides these principal chains the veins 

 form a plexus on the surface of the cord. Laterally branches 



