In boils sexes the lower extremity of the rectum is surrounded by the ramifications of the hcentorrhoidal plexus, the blood of which is returned to the internal iliac vein by the middle and inferior heemorrhoidal veins. This plexus likewise communicates freely with the superior hemorrhoidal veins, which unite with the inferior meseraic veins, and so constitute a part of the portal system.
In all these vessels valves exist in great numbers, although none occur in the great trunks in which they terminate.
The internal iliac vein on each side is in ternal to its corresponding artery.
The common iliac veins are formed by the union of the external and internal iliac veins. The junction takes place opposite the sacro iliac symphisis. These veins exhibit on each side some interesting anatomical peculiarities.
The right common iliac vein, nearly vertical in its direction, is placed posterior and external, to its corresponding artery.
The left common iliac vein, larger than the preceding, and nearly transverse in its direction, lies internal, and inferior, to its corresponding artery, and crosses behind the common iliac artery of the right side.
The common iliac veins usually unite upon the intervertebral substancebetween the fourth and fifth lumbar vertebrae, to form the yens cava inferior. The junction occurs to the right of the mesial line, and inferior to, as well as to the right of, the angle of bifurcation of the aorta.* The collateral branches of the common iliac veins are the following : (a) The ilio-lumbar vein arises in the iliac fossa, by radiating branches, which correspond in their distribution to those of the artery of the same name, and communicate freely with the lower lumbar and sacral veins.
(b) The middle sacral vein. — A single trunk placed on the sacrum in the middle line, which communicates inferiorly with the vesi cal and hremorrhoidalplexuses, and on each side with the adjacent lateral sacral veins. It usually opens into the left common iliac vein, but sometimes terminates more symmetrically, by bifurcating and giving a branch to either common iliac vein.
(c) The lateral sacral veins. — These ana stomose very freely with the middle sacral and glutaeal veins, and with veins in the sacral canal, before terminating in the common iliac veins.
Inferior yew eava. — (V ena cava ascen dens.) This vein, the largest in the body, since it returns to the heart all the blood which is circulated below the diaphragm, is formed by the junction of the common iliac veins, just as the superior cava is constituted by that of the venm innominatx. The abdominal vena cava passes upwards in front of the lumbar vertebra;, behind the liver and on the right side of the aorta. At first the inferior vena cava and the aorta are in close contact ; but as they ascend, the vein inclines forwards and to the right side, so that in the vicinity of the dia phragm these great blood-vessels become more and more remotely related to each other, and finally, the thoracic duct, the vena az3gos, the right splanchnic and sympathetic nerves, the right eras of the diaphragm and the Spigeliau lobe of the liver intervene between them. The anterior relations of the inferior vena cava are the following : the peritoneum, the mesentery, the inferior portion of the duodenum, the pancreas, the commencement of the vena portm, and the liver, which latter sometimes forms a complete canal for the cava. Pos teriorly, the inferior cava corresponds to the vertebrm, to the psoas muscle, and to the right renal and right lumbar arteries : exter nally, it is related to the right kidney.
The inferior vena cava is not uniform in its dimensions, for it presents two remarkable &illations in its course, the ,first where it is joined by the emulgent veins, and the second, opposite to its junction with the vena; cavw hepatica:.
flaying passed behind, or through, the liver, the vena cava is transmitted through the !` foramen quadratum " of the diaphragm, and at the same time penetrates the fibrous layer of the pericardium, which is here intimately connected with the cordiform tendon of the diaphragm. Above the diaphragm the vena cava bends abruptly to the left, and, after a very short, and almost transverse course within the pericardium, during which it is in vested on its anterior surface by the serous membrane, terminates by entering the most posterior and inferior portion of the right auricle ; the axis of its opening is directed upwards, backwards, and to the left side. There are no valves in this vessel.