THE SPINAL VEINS. 613 



veins in the neck, the intercostal veins in the thorax, and the lumbar and sacral 

 veins in the loins and pelvis. 



2. The Meningo-rachidian Veins. The principal veins contained in the spinal 

 canal are situated between the theca vertebralis and the vertebrae. They consist 

 of two longitudinal plexuses, one of which runs along the posterior surface of the 

 bodies of the vertebrae (anterior longitudinal spinal veins). The other plexus 

 (posterior longitudinal spinal veins) is placed on the inner or anterior surface of 

 the laminae of the vertebrae. 



The Anterior Longitudinal Spinal Veins consist of two large, tortuous veins 

 which extend along the whole length of the vertebral column, from the foramen 

 magnum, where they communicate by a venous ring around that opening, to the 

 base of the coccyx, being placed one on each side of the posterior surface of the 

 bodies of the vertebrae along the margin of the posterior common ligament. 

 These veins communicate together opposite each vertebra by transverse trunks 

 .which pass beneath the ligament, and receive the large vence basis vertebrarum 

 from the interior of the body of each vertebra. The anterior longitudinal spinal 

 veins are least developed in the cervical and sacral regions. They are not of 



FIG. 333. Vertical section of two dorsal vertebrae, showing the spinal veins. 



uniform size throughout, being alternately enlarged and constricted. At the 

 intervertebral foramina they communicate with the dorsi-spinal veins, and with 

 the vertebral veins in the neck, with the intercostal veins in the dorsal region, 

 and with the lumbar and sacral veins in the corresponding regions. 



The Posterior Longitudinal Spinal Veins, smaller than the anterior, are 

 situated one on each side, between the inner surface of the laminae and the theca 

 vertebralis. They communicate (like the anterior) opposite each vertebra by 

 transverse trunks, and with the anterior longitudinal veins by lateral transverse 

 branches which pass from behind forward. These veins, by branches which per- 

 forate the ligamenta subflava, join with the dorsi-spinal veins. From them 

 branches are given off which pass through the intervertebral foramina and join 

 the vertebral, intercostal, lumbar, and sacral veins. 



3. The Veins of the Bodies of the Vertebrae (vence basis vertebrarum) emerge 

 from the foramina on their posterior surface, and join the transverse trunk con- 

 necting the anterior longitudinal spinal veins. They are contained in large, tor- 

 tuous channels in the substance of the bones, similar in every respect to those 

 found in the diploe of the cranial bones. These canals lie parallel to the upper 

 and lower surface of the bones. They commence by small openings on the front 

 and sides of the bodies of the vertebrae, through which communicating branches 

 from the veins external to the bone pass into its substance, and converge to the 

 principal canal, which is sometimes double toward its posterior part, and open 

 into the corresponding transverse branch uniting the anterior longitudinal veins. 

 They become greatly developed in advanced age. 



4. The Veins of the Spinal Cord (medulli-spinaT) consist of a minute, tortuous, 



