THE INFERIOR CAVAL SYSTEM. 905 



veins of this smaller and posterior group enlarge to carry on the return circulation. 

 Elevation is of especial value in testicular iniiammation, as the dependent position of 

 the spermatic veins and their lack of adequate support greatly intensify the engorge- 

 ment and venous obstruction of inflammatory processes. 



The Common Iliac Veins. 



The common iliac veins (vv. iliacae communes) (Fig. 765) are two in number, 

 and are formed opposite the sacro-iliac articulations by the union of the internal and 

 external iliac veins. They pass upward, converging as they go, and unite at abou^. 

 the level of the intervertebral disk between the fourth and fifth lumbar vertebrae to 

 form the vena cava inferior. 



Since their point of union lies somewhat to the right of the median line, the right 

 vein is shorter than the left and its course is more directly upward. Neither vein 

 possesses valves. 



Relations. — The union of the two veins takes place beneath the right common 

 iliac artery, and the right vein, at its origin, lies behind that vessel, although, since its 

 course is more vertical than that of the artery, it gradually comes to lie somewhat 

 lateral to it above. The left vein near its termination is crossed from without inward 

 by the right common iliac artery, and throughout its course lies medially to the left 

 common iliac artery and on a plane somewhat posterior to it. 



Variations. — Occasionally the external and internal iliac veins do not unite to form a common 

 stem, but open directly into the inferior vena cava. This may occur on one or both sides. 



Tributaries. — In addition to the external and internal iliacs, by whose union 

 they are formed, the common iliacs receive but a single tributary, the middle sacral 

 vein (v. sacralis media), and this opens into the left vein. It accompanies the middle 

 sacral artery, and in the lower part of its course it is frequently double, one vessel 

 lying on each, side of the artery. Opposite each sacral vertebra it receives a 

 transverse connecting branch from the lateral sacral veins and so forms with these 

 what is termed the anterior sacral plexus. At its origin it communicates with 

 the hemorrhoidal veins. 



The Internal Iliac Vein. 



The internal iliac vein (v. hypogastrica) (Fig. 767) of each side is a short but 

 rather large vessel, which accompanies the internal iliac artery, lying to its medial 

 side and in a plane somewhat posterior to it. It extends from the neighborhood of 

 the great sacro-sciatic foramen to the level of the sacro-iliac synchondrosis, where it 

 unites with the external iliac to form the common iliac vein. 



Tributaries. — Its tributaries correspond in general with the branches of the 

 internal iliac artery, but those which arise in the pelvic viscera present the peculiarity 

 that they take their origin from more or less extensive plexuses which communicate 

 with one another. The stems which pass from these plexuses to the internal iliac 

 also anastomose to a considerable extent, the result being that it is not possible in all 

 cases to recognize definite veins corresponding to the visceral arteries. 



The following are the tributaries that are, as a rule, to be recognized : (i) the 

 gluteal, (2) the lateral sacral, (3) the ilio-lumbar, (4) the sciatic, (5) the internal 

 pndic, (6) the obturator, (7) the middle hemorrhoidal, (8) the uterine, and (9) the 

 vesical veins. 



I. The Gluteal Vein, — The gluteal vein (v. glutaea superior) accompanies the 

 artery of the same name. Throughout its extrapelvic course its tributaries accom- 

 pany the branches of the artery as valved venae comites, and at the upper part of the 

 greater sacro-sciatic foramen the veins accompanying the two main branches of the 

 artery unite to form a double trunk, united by numerous anastomoses. This trunk, 

 which is occasionally single, passes through the greater sacro-sciatic foramen above 

 the pyriformis muscle and, after a short intrapelvic course, opens into the internal 

 iliac vein. 



Where they pass through the greater sacro-sciatic foramen both artery and vein 

 are surrounded by a dense connective tissue which renders their separation difificult 

 and brings it about that the lumen of the vein remains patent when emptied of blood. 



