OF THE LOWER EXTREMITY. 



551 



veins in the dorsal region, and with the lumbar and sacral veins in the corres- 

 ponding regions. 



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

 ated one on either 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 trans- 

 verse branches, which pass from behind forwards. These veins, at the inter- 

 vertebral foramina, join with the dorsi-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. 



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 communi- 



Fig. 310. Vertical Section of two Dorsal Vertebrae, showing the Spinal Veins. 



cate with the vertebral 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 intervertebral foramina, where they join the other veins 

 from the spinal canal. 



There are no valves in the spinal veins. 



VEINS OF THE LOWER EXTREMITY. 



The Veins of the Lower Extremity are subdivided, like those of the upper, 

 into two sets, superficial and deep; the superficial veins being placed beneath 

 the integument, between the two layers of superficial fascia ; the deep veins 

 accompanying the arteries, and forming the vena3 comites of those vessels. 

 Both sets of veins are provided with valves, which are more numerous in the 

 deep than in the superficial set. These valves are also more numerous in the 

 lower than in the upper limb. 



The Superficial Veins of the lower extremity are the internal or long saphenous, 

 and the external or short saphenous. 



