898 HUMAN ANATOMY. 



ThQ posterior' internal plexuses are situated one on either side of the median line 

 on the anterior surfaces of the lamince and on the Hgamenta subflava, through which 

 they send branches to communicate with the posterior external plexus. They are 

 connected by transverse plexuses which complete the retia venosa vertebrarum, and 

 are composed of smaller vessels than the anterior plexuses, and the net-work ^^•hich 

 they form is more open. 



Laterally, at each intervertebral foramen the internal plexuses send branches out 

 from the spinal canal along the nerve-trunks, and by means of these intervertebral 

 veins (w. inten'ertebrales), which have the form of plexuses at their origin and receive 

 communicating branches from the external vertebral plexuses and from the veins of the 

 spinal cord, the internal plexuses pour their blood into the vertebral, intercostal, lum- 

 bar, and lateral sacral veins, the connection with the intercostals being through their rami 

 spinales. Above, the internal plexuses form an especially rich rete or plexus around 

 the foramen magnum and communicate with the occipital, marginal, and basilar sinuses. 



Practical Considerations. — The posterior external spinal plexuses, by means 

 of their communication through the intervertebral foramina and the ligamenta sub- 

 flava with the internal plexuses, may convey infection from without — septic wounds 

 of the back, severe bed-sores, osteitis of the vertebral laminae — to the interior of the 

 spinal canal. External pachymeningitis has thus originated. In operations upon 

 the spine, these veins bleed so freely that it is often well after severing them upon 

 one side to control them by packing and proceed to the exposure of the spine on 

 the opposite side, repeating the packing there. The internal plexuses, interposed 

 between the theca of the cord and the interior walls of the vertebral column, may, 

 as a result of trauma, furnish blood enough to cause compression of the cord. The 

 symptoms are usually relatix-ely slow in developing — as compared with those due to 

 injury to the cord itself or to its vessels — and are referable mainly to the lower spinal 

 segments, the blood gravitating to that portion of the canal. 



Hemorrhage may occur within the membranes (haematorrhacis), when the 

 blood will likewise tend to gravitate toward the lower end of the cord, and, unless 

 in large amount, may cause no definitely localizing symptoms. Bleeding from the 

 venae medulli spinales may take place into the substance of the cord (haemato- 

 mvelia), and is most likely to occur in the segments from the fourth cervical to the 

 first dorsal (Thorburn), because of the degree of motion of that portion of the spine, 

 the union toward its base of a fixed and a movable segment, and the frequency with 

 which forces causing excessive flexion or over-extension are applied to the head. 

 If the lesion causes compression only, the paralysis, anaesthesia, etc. , will be only 

 temporary. If it is associated with disorganization of the cord, they will be permanent. 



The Veins of the Spinal Cord. 



The veins of the spinal cord (vv. medulli spinales) occur as six longitudinal stems 

 situated upon the surface of the cord and connected by a fine net-work very much as 

 are the arteries. One of these stems traverses the entire length of the cord along the 

 line of the anterior median fissure, and has on either side of it another stem which 

 lies immediately posterior to the line of exit of the anterior nerve-roots. These three 

 stems together form the anterior medulli-spinal veins (vv. spinales externae 

 anteriores). The posterior veins (w. spinales externae posteriores) have a similar 

 arrangement, one lying along the line of the posterior longitudinal fissure and one 

 posterior to each of the lines of entrance of the posterior nerve-roots. 



All these stems, together with the plexus which connects them, lie in the pia 

 mater and receive branches (vv. spinales internae) from the substance of the cord. 

 From them branches pass out along the nerve-roots to join the intervertebral veins, 

 and at the upper extremity of the cord they join the veins of the medulla oblongata. 



THE INFERIOR CAVAL SY.STEM. 



The inferior caval system includes all the veins from the body-wall below the 

 level of the diaphragm ; those from the abdominal and pelvic cavities, with the 

 exception of those from the stomach, intestines (except the lower part of the rectum), 



