550 VEINS. 



1. Those placed on the exterior of the spinal column (the dorsi-spinal veins). 



2. Those situated in the interior of the spinal canal, between the vertebree 

 and the theca vertebralis (meningo-rachidian veins). 



3. The veins of the bodies of the vertebrae. 



4. The veins of the spinal cord (medulli-spinal). 



1. The Dorsi-spinal Veins commence by small branches, which receive their 

 blood from the integument of the back of the spine, and from the muscles in 

 the vertebral grooves. They form a complicated network, which surrounds 

 the spinous processes, the laminae, and the transverse and articular processes 

 of all the vertebras. At the bases of the transverse processes, they communi- 

 cate, by means of ascending and descending 'branches, with the veins surround- 

 ing the contiguous vertebras, and they join with the veins in the spinal canal 

 by branches which perforate the ligamenta subflava ; they terminate in the 

 intervals between the arches of the vertebrae, by joining the vertebral veins in 

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

 in the loins and pelvis. 



2. 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 vertebras 

 throughout the entire length of the spinal canal (anterior longitudinal spinal 

 veins), receiving the veins belonging to the bodies of the vertebras (venas basis 

 vertebrarum). The other plexus (posterior longitudinal spinal veins) is placed 

 on the inner, or anterior surface of the laminae of the vertebras, and extends also 

 along the entire length of the spinal canal. 



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

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

 foramen magnum to the base of the coccyx, being placed one on each side of 

 the posterior surface of the bodies of the vertebras, external to the posterior 

 common ligament. These veins communicate together opposite each vertebra, 

 by transverse trunks, which pass beneath the ligament, and receive the large 

 venas basis vertebrarum, from the interior of the body of each vertebra. The 



Fig. 309. Transverse Section of a Dorsal Vertebra, showing the Spinal Veins. 



anterior longitudinal spinal veins are least developed in the cervical and sacral 

 regions. They are not of 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 



