THE AZYGOS SYSTEM. 897 



111 the two or three lower veins there is no such double flow, the valves all 

 looking towards the azygos veins. Valves occur at the opening 01 practically all the 

 intercostals into the azygos veins, the last intercostal forming an exception to this 

 rule, and, furthermore, the valves of the lower veins are apt to be insufficient. 



The Spinal Veins. 



The spinal veins, which return the blood from the vertebral column and the 

 adjacent muscles and also from the membranes enclosing the spinal cord, present in a 

 high degree the plexiform arrangement which is characteristic of the veins as com- 

 pared with the arteries. They form a series of longitudinal plexuses which extend 

 practically the entire length of the spinal column, communicating extensively with one 

 another, and may be divided primarily into those which lie external to the spinal 

 canal and those which lie within the canal. 



The external spinal plexuses (plexus venosi vertebrales externi) are two in 

 number, anterior and posterior. The anterior external plexus (plexus venosus 

 vertebralis anterior) rests upon the anterior surfaces of the bodies of the vertebrae, 

 and presents considerable differences in the amount of its complexity in different 

 portions of the spinal column. In the thoracic and lumbar regions it forms a net- 

 work with large meshes, in the sacral region it is represented by transverse anasto- 

 moses between the lateral and middle sacral veins, and in the cervical region it reaches 

 its greatest degree of complexity, forming a close net-work, especially dense above 

 and resting partly upon the bodies of the vertebrae and partly upon the longus colli 

 muscles. At each intervertebral foramen the plexus communicates with the veins 

 issuing from the internal spinal plexuses and also with the posterior external plexus, 

 and in addition sends branches to the vertebral veins in the cervical region and to 

 the rami spinales of the intercostal and lumbar veins in the corresponding regions. 



The posterior external plexuses (plexus venosi vertebrales posteriores) lie partly 

 upon the posterior surfaces of the laminae of the vertebrae and the ligamenta subflava 

 and partly between the deeper dorsal muscles. As in the case of the anterior plexus, 

 they are more complicated in the cervical than in the thoracic and lumbar regions. 

 In the latter their meshes are somewhat elongated longitudinally, and they communi- 

 cate with the internal plexuses at the intervertebral foramina and also by branches 

 which traverse the ligamenta subflava, and they have further communications with 

 the anterior external plexus and with the spinal rami of the intercostal and lumbar 

 veins. In the cervical region, in correspondence with the greater differentiation of 

 the dorsal musculature, the plexuses become divided into several layers, and in the 

 region between the occiput and the axis vertebra their deep layers form an especially 

 dense net-work, the suboccipital plexus, with which the occipital, vertebral, deep 

 cervical, and posterior external jugular veins communicate. Throughout its course 

 the cervical portion of the plexus communicates with the internal and anterior ex- 

 ternal plexuses and also with the vertebral vein. 



The internal spinal plexuses (plexus venosi vertebrales interni) are situated in 

 the dura mater lining the spinal canal and are much closer than the external plexuses. 

 The veins which form them have a general longitudinal direction and anastomose 

 abundantly, but nevertheless four subordinate longitudinal lines of vessels can be 

 recognized, two of which are upon the anterior wall of the spinal canal and two upon 

 the posterior wall. 



The anterior internal plexuses lie one on each side of the median line on the 

 posterior surfaces of the bodies of the vertebrae and the intervertebral disks, from the 

 foramen magnum to the sacral region. They are composed of rather large veins, 

 between which are frequent anastomoses, and transverse connecting vessels run across 

 the body of each vertebra between the two plexuses, passing beneath the posterior 

 common vertebral ligament. Into these transverse connections open the basivertebral 

 veins (vv. basivertebrales) which return the blood from the bodies of the vertebrae, 

 traversing these to a certain extent to communicate with the anterior external ple.xus. 

 The anterior internal plexuses also communicate opposite each vertebra with the pos- 

 terior internal plexuses, rings of anastomosing veins thus surrounding the spinal canal 

 opposite each vertebra and constituting what are termed the retia venosa vertebrarum. 



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