630 THE VK/XS 



The anterior longitudinal spinal veins extend from the foramen magnum to 

 the coccyx as two tortuous })lcxiform vest^els, one being placed on each side of the 

 liack of the boches of the vertelirte behind the posterior common hgament. Oppo- 

 site the body of each vertebra they connnunicate V)y a transverse Ijranch, which 

 passes between the body of the vertebra and posterior eonnnon Hgaiiient, an 

 arrangement which is sometimes spoken of as the spinal venous ladder. Each 

 transverse branch as it lies under cover of the posterior common ligament receives 

 the veins from the bodies of the vertebrae (the venae basis vertebrae). At the 

 spot where each longitudinal vein is joined by the transverse l>ranch, the vessel 

 becomes considerably dilated. From the longitudinal vein branches run backwards 

 to join the posterior longitudinal veins, and op})osite the intervertebral foramina a 

 transverse branch runs outwards to join the vertebral vein, the intercostal veins, the 

 lumbar veins, and the sacral veins, according to the region of the si)ine in ^\•llich 

 the vertebra is situated. Above, the anterior spinal veins communicate with the 

 l)asilar plexus at the front of tlie foraim-n magnum. 



The posterior longitudinal spinal veins, smaller tlian the anterior longitudinal 

 veins, likewise extend from the cranium to the coccyx. They lie between the pos- 

 terior wall of the spinal canal and the dura mater. Like the anterior, they commu- 

 nicate by transverse branches, which receive veins through the ligamenta subflava 

 from the dorsal spinal plexus. They also communicate with the anterior longi- 

 tudinal veins by lateral transverse branches. 



It will be thus seen (fig. 390) that in the interior of the vertebral canal, opposite 

 each vertebral segment, there is a venous ring between the bony wall of the canal 

 and the sheath of the dura mater, the ring being formed in front by the anterior 

 transverse vein; on each side by the dilated portion of the trunk of the anterior 

 longitudinal spinal vein, and the lateral transverse lu-anch; and behind, by the 

 trunk of the posterior longitudinal vein and the posterior transverse branch. This 

 venous ring receives veins from the body of the vertebra, from the spinal cord, and 

 from the meninges, and pours its blood, in part through the lateral veins lying in 

 the intervertebral foramina into the vertebral, intercostal, lumliar, or sacral veins; 

 and in part through the branch which perforates the ligamenta subflava into the 

 dorsal spinal plexus. Above, the posterior longitudinal veins connnunicate at the 

 back of the foramen magnum with the occipital sinuses. Around the foramen 

 magnum a distinct venous ring or plexus is formed l)y the connnunication 

 between the occipital and marginal sinuses and the posterior and anterior spinal 

 veins. 



(6) The medullary or medulli-spinal veins, or veins of the spinal cord, are 

 of small size, and run in the pia mater in a tortuous course along the spinal cord. 

 They join the venous ring corresponding to each vertebral segment by passing along 

 the sheath of dura mater reflected round the spinal nerves. 



4. THE VEIXS OF THE ABDOMEN AND PELVIS 



All the veins of the al)domen and pelvis — Avith the exception of the superior 

 epigastric vein and ascending lum])ar vein, which open ultimately into the su})erior 

 vena cava — enter directly or indirectly into the inferior vena cava. The veins 

 corres])onding to the parietal branches of the abdominal aorta, except the middle 

 sacral vein, open directly into the inferior vena cava; the middle sacral vein only 

 indirectly through the left common iliac vein. Of the visceral veins corres]ionding 

 to the visceral branches of the abdominal aorta, those which return the blood from 

 the stomach, intestines, and pancreas (the chylopoetic viscera), and from the 

 spleen, end in a common trunk (the portal vein). The portal vein enters tlie 

 liver, and breaks up in the liver substance into cai)illaries like an artery, and from 



