THE SPIXAL YEIXS. 493 



down by tlie branches of communication wliich pass outwards. In 

 some parts the veins are double, or even triple, so as to form a plexus, 

 and occasionally they are altogether interrupted. In the thoracic 

 region their communicating branches open into the intercostal veins, 

 in the loins inio the lumbar veins, in the neck for the most part into 

 the veiieljral. 



d. The posterior longitudinal spinal veins are a complex interlace- 

 ment of tortuous veins along the inner or anterior surface of the arches 

 of the vertebrae. In the lower part of the canal this interlacement of 

 veins is not so close as in the upper portion, Avhere it usually conceals 

 (if the injection has been successful) the whole surface of the dura 

 mater. These veins converge to the intervertebral foramina, and join 

 by rather small vessels with the intercostal veins. 



c. — The veins of the spinal cord (Breschet) ramify upon tlie cord 

 and its nerves, enclosed within the sheath formed by the dura mater. 

 Thougli they communicate with the other spinal veins, they are not 

 injected with them, even when the injecting process is most successful. 

 Very small, 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 smaher 

 than in the lumbar region. They communicate freely with the spinal 

 veins and plexuses, by means of branches which accompany the nerves 

 towards the intervertebral foramina. Near the base of the skull they 

 unite to form two or three small trunks, which communicate by trans- 

 verse 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 jSow through them horizontally. 

 The dorsal veins pour their blood into the longitudinal plexus on the 

 inner surface of the arches of the vertebra ; thence it is collected, at 

 each of the 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. 



INPEmOR VEJSTA 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, and inclines forwards to 

 reach the_ opening in the diaphragm appropriated to it, and after beiug 

 enclosed in a fold of the pericardium, terminates in the right auricle 

 of l;he heart. A large valve is situated at its entrance into the 

 auricle, named the valve of Eustachius, which, however, as explained 

 in the description of the heart, is only a vestige of a foetal structure, 

 variable in size, and without influence in preventing reflux of the 

 blood. 



Tributaeies. — Besides the common iliac veins, the inferior vena 

 cava receives the following. 



