534- THE INFERIOR VENA CAVA. 



meshes. They are larger below than above, and on the posterior than on the 

 anterior surface of the cord. One considerable trunk lies beneath the anterior spinal 

 artery over the anterior median fissure of the cord, and another more or less regular 

 vessel, or anastomotic chain, is found on each side immediately behind the anterior 

 nerve-roots. On the posterior surface there is also a median longitudinal vein, 

 which is largest and most constant in the cervical and lumbo-sacral regions. These 

 vessels communicate with the veins of the spinal canal by means of branches which 

 are more numerous than the medullary arteries, and which accompany the nerve- 

 roots to the intervertebral foramina ; and at the upper end they are prolonged to 

 the bulb, where they join the veins of the pons and cerebellum, and give offsets to 

 the sinuses around the foramen magnum. 1 



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 flow 

 through them horizontally in the rings that are formed by the transverse branches 

 between the longitudinal veins, and the offsets proceeding from the latter to the 

 intervertebral foramina. The veins issuing from the spinal canal open, according to 

 the region in which they are placed, into the vertebral .veins, into the posterior 

 branches of the intercostal and lumbar veins, and into the lateral sacral veins. 



LNTERIOB, VENA CAVA. 



The inferior or ascending vena cava returns the blood from the lower limbs and 

 abdomen. It begins at the junction of the two common iliac veins in front of the 

 right half of the fifth lumbar vertebra, and thence ascends along the right side of 

 the aorta, being covered by the duodenum, pancreas, and lower end of the portal 

 vein, to the posterior surface of the liver ; there it becomes embedded in a deep 

 groove, not unfrequently a canal, in that organ, and inclines forwards to reach its 

 opening in the tendon of the diaphragm, to the margin of which the wall of the 

 vessel is firmly united. After perforating the diaphragm, it is enclosed for a very 

 short distance in a fold of the serous layer of the pericardium, and then terminates 

 by entering the right auricle of the heart, about the level of the disc between the 

 eighth and ninth dorsal vertebrae. A semilunar valve, known as the valve of 

 Eustachius, is situated over its entrance into the auricle, but this, as explained in 

 the description of the heart, is only the vestige of a 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 : 



1. The lumbar veins (fig. 411, p. 531) correspond in number with the arteries 

 of the same name. They are formed by the junction of anterior branches from the 

 wall of the abdomen, where they communicate with the epigastric and other neigh- 

 bouring veins, and posterior branches, of larger size, which receive tributaries from 

 the muscles of the back, the dorsal spinal plexus, and the spinal canal. Passing 

 forwards upon the bodies of the vertebrae, beneath the psoas muscle, and on the left 

 side also behind the aorta, they terminate by opening into the back of the inferior 

 vena cava. Two of the vessels, either of the same or of opposite sides, may join 

 together into a single trunk before their termination. The lumbar veins of each 

 side communicate with one another by means of branches which cross in front of 

 the transverse processes, and in this way a longitudinal vessel is formed, called the 

 ascending lumbar vein, which connects together more or less completely the lateral 

 sacral, ilio-lumbar, common iliac, and lumbar veins, and is continued upwards into 

 the corresponding azygos vein. 



1 See A. Adamkiewicz and H. Kadyi, opp. cit. on p. 421. 



