2. The anterior longitudinal sinuses, Willis : (grandes veines rachidiennes longitudinales an terieures, Breschet), [vide Fig. 360. p.630.] —These extend the entire length of the ver tebral column, under the form of two longi tudinal veins situated within the spinal canal, along the external margins the posterior common ligament, and consequently between the bodies of the vertebrx and the dura mater, (hence sometimes termed meningo rachidian veins). Opposite every vertebra these longitudinal vessels are connected to each other by the transverse plexus of the basi-vertebral veins; whilst externally they communicate, by means of branches which pass through the series of intervertebral fora mina, with the numerous veins which ramify on the exterior of the spinal column, viz., the vertebral, azygos, intercostal, lumbar, sacra], &c. These longitudinal venous channels are neither parallel to each other, nor are they of uniform dimensions; opposite the bodies of the vertebrae they are most closely approxi mated, whilst corresponding to the interver tebral foramina they are widely separated, as if drawn outwards by the branches which are here connected with them. Each longitudinal sinus, or " venus plexus," (for occasionally two or more veins enter into the formation of these channels on each side,) might therefore be described as formed of " a series of plexi form arches, which embrace the pedicles of each vertebra, have their concavity directed outwards and their convexity inwards, and the extremities of which anastomose together opposite the intervertebral foramina, where they communicate with the branches on the outside of the spine."# According to Bres chet, interruptions occasionally occur in dif ferent parts of these longitudinal channels, a circumstance which still further authorises this description, in which each venous arch is regarded as a separate trunk, communicating with its fellows of the opposite side, and also with similar branches above and below.
3. The posterior deep spinal veins (veines longitudinales rachidiennes posterieures, Bres chet) are likewise situated in the interior of the spinal canal, between the posterior sur face of the dura mater and the front of the laminm. They there form a close interlace
ment of vessels (posterior plexuses) which is most remarkably developed in the upper part of the canal. These veins and plexuses are joined by the posterior superficial spinal veins veins), and they com municate with the anterior longitudinal sinuses by numerous small lateral branches.
4. The proper veins (f the spinal cord (me dulli spinales, Breschct) are small tortuous vessels, which form an irregular plexus upon both surfaces of the medulla spinalis between the pia mater and the arachnoid membrane ; they communicate through the foramen mag num with the petrosal sinuses or cerebellar veins, and give off small branches which pass through the foramina with the spinal nerves to establish communications with the several veins. The veins of the spinal cord are apparently the " venw comites " of the proper spinal arteries.
" The veins of due spine may be regarded, • reference to the general circulation, as tablishing an unbroken communication tween the veins of all parts of the trunk, so that we can suppose one of the vente cavle to be obliterated without the venous circulation being interrupted. The greater azygos itself, which is generally regarded as the principal means of communication between the two venee cavm, is not, however, necessary, when we consider the arrangement of the anterior and posterior spinal plexuses. Thus, I have sometimes seen the inferior, and sometimes the superior vena cave, obliterated without any apparent increase in the diameter of the vena azygos, and, what will perhaps be thought surprising, without cederna either of the upper or lower extremities.
" Supposing the versa cava ascendens to be obstructed from the entrance of the hepatic veins down to the renal veins, the blood would then flow back by the lumbar veins into the plexuses contained within the spinal canal ; these plexuses it would ascend to the vertebro-costal (intercostal) veins, from thence to the azygos veins, and through them into the superior vena cava.
" If all the jugular veins were obliterated, the venous circulation in the head would still continue, and would be carried on through the spinal veins." *