LOWER VEXA CAY A. 473 



long, and tortuous, they run upon both, surfaces of the cord, and form a 

 diffused network. They become larger, for the most part, as they ascend, 

 but near the base of the skull they are smaller than in the lumbar region. 

 They communicate freely with the spinal veins and plexuses, by means of 

 branches which accompany the nerves towards the inter vertebral foramina. 

 Near the base of the skull they unite to form two or three small trunks, 

 which communicate by transverse branches with the vertebral veins, and 

 terminate in the inferior cerebellar veins, or in the petrosal sinuses. 



From a consideration of the connection and arrangement of the different 

 parts of these complex veins, it would appear that the main currents of the 

 blood in each part flow through them horizontally. The dorsal veins pour 

 their blood into the longitudinal plexus on the inner surface of the arches of 

 the vertebrae ; thence it is collected, at each of tha intervertebral foramina, 

 by two or three small converging branches, which open into some of the 

 veins outside the vertebral column in front, viz., into the lumbar, azygos, 

 and cervical veins. Into these, also, the contents of the great spinal veins 

 are conveyed by the short communicating branches already noticed. 



LOWER VENA CAVA. 



The lower or ascending vena cava returns the blood from the lower limbs, 

 and from the viscera of the pelvis and abdomen. It commences at the 

 junction of the two common iliac veins on the side of the fifth lumbar 

 vertebra, and thence ascends along the right side of the aorta, as far as the 

 posterior border of the liver ; it there becomes lodged in a groove in that 

 organ, after which it inclines forwards to reach the opening in the diaphragm 

 appropriated to it, and, after being enclosed in a fold of the pericardium, 

 terminates in the right auricle of the heart. A large valve is situated at 

 its entrance into the auricle, named the valve of Eustachius, which, however, 

 as already explained in the description of the heart, is only a vestige of 

 foetal structure, variable in size, and without influence in preventing reflux 

 of the blood. 



TRIBUTARIES. Besides the common iliac veins, the inferior vena cava 

 receives the following. 



a. The middle sacral vein, taking its course upwards on the front of the 

 sacrum, opens into the left common iliac vein, or into the commencement of 

 the vena cava. 



b. The lumbar veins correspond in number with the arteries of the same 

 name : they commence by small dorsal branches in the muscles of the back ; 

 and by others from the walls of the abdomen, where they communicate 

 with the epigastric and other veins in the neighbourhood. Having reached 

 the spine, they receive branches from the spinal plexuses, and proceed forward 

 upon the bodies of the vertebrae, behind the psoas muscle : those on the left 

 side, passing behind the aorta, terminate in the back of the vena cava. 

 Some of these veins are frequently found to unite into a single trunk before 

 their termination. The lumbar veins of the same side communicate with 

 each other by branches which cross in front of the transverse processes. 

 Not unfrequently a branch of this description is met with, called the 

 ascending lumbar vein, which connects more or less completely the common 

 iliac vein, the ilio-lumbar and lumbar veins, and the azygos vein. 



c. The spermatic veins, proceeding upwards from the testicle and 

 forming a part of the constituents of the spermatic cord, enter the abdomen, 

 and ascend on the psoas muscle behind the peritoneum. Below the 



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