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ANGIOLOGY 



along the entire length of the column; these plexuses may be divided into two 

 groups, external and internal, according to their positions inside or outside the 

 vertebral canal. The plexuses of the two groups anastomose freely with each other 

 and end in the intervertebral veins. 



The external vertebral venous plexuses (plexus venosi vertebrates externi; extra- 

 spinal veins') best marked in the cervical region, consist of anterior and posterior 

 plexuses which anastomose freely with each other. The anterior external plexuses 

 lie in front of the bodies of the vertebrae, communicate with the basivertebral and 

 intervertebral veins, and receive tributaries from the vertebral bodies. The pos- 

 terior external plexuses are placed partly on the posterior surfaces of the vertebral 

 arches and their processes, and partly between the deep dorsal muscles. They are 

 best developed in the cervical region, and there anastomose with the vertebral, 

 occipital, and deep cervical veins. 



The internal vertebral venous plexuses (plexus venosi vertebrales interni; intra- 

 spinal veins') lie within the vertebral canal between the dura mater and the verte- 

 brae, and receive tributaries from the bones and from the medulla spinalis. They 



Posterior external plexuses 





Fia. 578. Transverse section of a thoracic vertebra, 

 showing the vertebral venous plexuses. 



Fia. 579. Median sagittal section of two thoracic verte- 

 brae, showing the vertebral venous plexuses. 



form a closer net-work than the external plexuses, and, running mainly in a vertical 

 direction, form four longitudinal veins, two in front and two behind; they therefore 

 may be divided into anterior and posterior groups. The anterior internal plexuses 

 consist of large veins which lie on the posterior surfaces of the vertebral bodies and 

 intervertebral fibrocartilages on either side of the posterior longitudinal ligament; 

 under cover of this ligament they are connected by transverse branches into which 

 the basivertebral veins open. The posterior internal plexuses are placed, one on 

 either side of the middle line in front of the vertebral arches and ligamenta flava, 

 and anastomose by veins passing through those ligaments with the posterior exter- 

 nal plexuses. The anterior and posterior plexuses communicate freely with one 

 another by a series of venous rings (retia venosa vertebrarum) , one opposite each 

 vertebra. Around the foramen magnum they form an intricate net-work which 

 opens into the vertebral veins and is connected above with the occipital sinus, 

 the basilar plexus, the condyloid emissary vein, and the rete canalis hypoglossi. 



The basivertebral veins (vv. basivertebrales) emerge from the foramina on the 

 posterior surfaces of the vertebral bodies. They are contained in large, tortuous 



