472 



VEINS. 



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

 which pass from behind forwards. These veins, at the intervertebral foramina, 

 join with the dorsi-spinal veins. 



Fig. 242. Transverse Section of a Dorsal Vertebra, showing the Spinal Veins. 



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

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

 connecting the anterior longitudinal spinal veins. They are contained in large, 

 tortuous 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, arise from the entire circumference of the vertebra, 

 communicate with veins which enter through the foramina, on the anterior surface 

 of the bodies, and converge to the principal canal, which is sometimes double 

 towards its posterior part. They become greatly developed in advanced age. 



Fig. 243. Vertical Section of two Dorsal Vertebrae, showing the Spinal Veins. . 



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

 venous plexus, which covers the entire surface of the cord, being situated between 

 the pia mater and arachnoid. These vessels emerge chiefly from the posterior 

 median furrow, and are largest in the lumbar region. Near the base of the skull 

 they unite, and form two or three small trunks, which communicate with the ver- 

 tebral veins, and then terminate in the inferior cerebellar veins, or in the petrosal 

 sinuses. Each of the spinal nerves is accompanied by a branch as far as the in- 

 tervertebral foramina, where they join the other veins from the spinal canal. 

 There are no valves in the spinal veins. 



