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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
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 vertebrales externi; extraspinal 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 vertebræ, communicate with the basivertebral and intervertebral veins, and receive tributaries from the vertebral bodies. The posterior 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.


FIG. 578– Transverse section of a thoracic vertebra, showing the vertebral venous plexuses. (See enlarged image)



FIG. 579– Median sagittal section of two thoracic vertebræ, showing the vertebral venous plexuses. (See enlarged image)

  The internal vertebral venous plexuses (plexus venosi vertebrales interni; intraspinal veins) lie within the vertebral canal between the dura mater and the vertebræ, and receive tributaries from the bones and from the medulla spinalis. They 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 external 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

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