ILIAC INFERIOR VENA CAYA. 475 



artery, and terminates opposite the sacro-iliac articulation, by uniting with the 

 external iliac, to form the common iliac vein. This vessel has no valves. 



The hemorrhoidal plexus surrounds the lower end of the rectum, being formed 

 by the superior hemorrhoidal veins, branches of the inferior mesenteric, and the 

 middle and inferior hemorrhoidal, which terminate in the internal iliac. The 

 portal and general venous systems have a free communication by means of the 

 branches composing this plexus. 



The vesico-prostatic plexus surrounds the neck and base of the bladder and 

 prostate gland. It communicates with the hemorrhoidal plexus behind, and 

 receives the dorsal vein of the penis, which enters the pelvis beneath the sub- 

 pubic ligament. This plexus is supported upon the sides of the bladder by a 

 reflection of the pelvic fascia. The veins composing it are very liable to become 

 varicose, and often contain hard earthy concretions, called phlebolites. 



The dorsal vein of the penis is a vessel of large size, which returns the blood 

 from the body of this organ. At first it consists of two branches, which are con- 

 tained in the groove on the dorsum of the penis, and receives veins from the 

 glans, the corpus spongiosum, and numerous superficial veins ; these unite near 

 the root of the penis into a single trunk, which pierces the Jriangular ligament 

 beneath the pubic arch, and divides into two branches, which enter the prostatic 

 plexus. 



The vaginal plexus surrounds the mucous membrane of the vagina, being espe- 

 cially developed at the orifice of this canal ; it communicates with the vesical 

 plexus in front, and with the hemorrhoidal plexus behind. 



The uterine plexus is situated along the sides and superior angles of the uterus, 

 receiving large venous canals (the uterine sinuses) from its substance. The veins 

 composing this plexus anastomose frequently with each other and with the ovarian 

 veins. They are not tortuous like the arteries. 



Each COMMON ILIAC VEIN is formed by the union of the external and internal 

 iliac veins in front of the sacro-vertebral articulation ; passing obliquely upwards 

 towards the right side, they terminate upon the intervertebral substance between 

 the fourth and fifth lumbar vertebrae, where they unite at an acute angle to form 

 the inferior vena cava. The right common iliac is shorter than the left, nearly 

 vertical in its direction, and ascends behind and then to the outer side of its 

 corresponding artery. The left common iliac, longer and more oblique in its course, 

 is at first situated at the inner side of the corresponding artery, and then behind 

 the right common iliac. Each common iliac receives the ilio-lumbar, and some- 

 times the lateral sacral veins. The left one receives, in addition, the middle sacral 

 vein. No valves are found in these veins. 



The middle sacral vein accompanies its corresponding artery along the front of 

 the sacrum, and terminates in the left common iliac vein; occasionally in the 

 commencement of the inferior vena cava. 



Peculiarities. The left common iliac vein, instead of joining with the right one in its usual 

 position, occasionally ascends on the left side of the aorta as high as the kidney, where, after 

 receiving the left renal vein, it crosses over the aorta, and then joins with the right vein to form 

 the vena cava. In these cases, the two common iliacs are connected by a small communicating 

 branch at the spot where they are usually united. 



The INFERIOR VENA CAVA returns to the heart the blood from all the parts below 

 the Diaphragm. It is formed by the junction of the two common iliac veins on 

 the right side of the intervertebral substance, between the fourth and fifth lumbar 

 vertebrae. It passes upwards along the front of the spine, on the right side of the 

 aorta, and, having reached the under surface of the liver, is contained in a groove 

 in its posterior border. It then perforates the tendinous centre of the Diaphragm, 

 enters the pericardium, where it is covered by its serous layer, and terminates in 

 the lower and back part of the right auricle. At its termination in the auricle, 

 it is provided with a valve, the Eustachian, which is of large size during foetal life. 



Relations. In front, from below upwards, with the mesentery, transverse por- 



