THE VISCERAL VENOUS PLEXUSES. 51-1 



from the corpora cavernosa and the skin of the organ. At the root of the penis the 

 dorsal vein passes through the aperture below the subpubic ligament (p. 338), form- 

 ing a communication on each side with the commencement of the pudic vein, and 

 then divides into two branches which enter the right and left portions of the pros- 

 tatic plexus. Each of these divisions is also connected with the obturator vein of 

 the same side by a considerable branch which ascend^ on the back of the pubis 

 towards the thyroid foramen. 



The prostatic plexus is formed mainly by the breaking up of the divisions of 

 the dorsal vein of the penis, but it receives also smaller branches from the 

 gland and the neighbouring muscles. It surrounds the base of the prostate, 

 most thickly on its anterior and lateral aspects, and communicates below with 

 the tributaries of the pudic vein, while above it is continuous with the vesical plexus. 

 In old persons these veins are generally much enlarged, and their valves become im- 

 perfect or disappear. 



In the female, a similar plexus surrounds the upper part of the urethra and re- 

 ceives the dorsal vein of the clitoris. 



The vesical plexus consists of vessels which ramify over the whole of the bladder 

 external to its muscular coat, being particularly large and numerous towards the base 

 of the organ, where they receive branches from the ureters, the vasa deferentia and 

 the vesiculas seminales, and are closely connected with the prostatic and hsemor- 

 rhoidal plexuses in the male, or with the vaginal plexus in the female. 1 



The hcemorrhoidal plexus consists of large and copiously anastomosing veins in 

 the wall of the lower part of the rectum, immediately underneath the mucous mem- 

 brane. From it proceed superior, middle, and inferior hcemorrhoidal veins correspond- 

 ing generally to the arteries of the same name, and it communicates freely with the 

 plexuses in front of it. The superior hasmorrhoidal vein being a branch belonging 

 to the portal system, the haemorrhoidal plexus forms a very free communication 

 between the portal and general venous systems. 



The vaginal plexus, surrounding the vagina principally in its lower part, com- 

 municates freely with the hsemorrhoidal and vesical plexuses. 



The uterine plexus pours its blood in greatest part into the ovarian veins, and is 

 not considerable except during pregnancy. 



THE PORTAL SYSTEM OF VEINS. 



The portal vein differs from other veins of the body in being subdivided into 

 branches at both its extremities. The branches of origin, by the union of which it 

 may be said to be formed, are the veins of the chylopoietic viscera (stomach, intestine 

 and pancreas) and of the spleen ; they run as single companion vessels with the 

 corresponding arteries and their offsets. The other branches, or those of distribu- 

 tion, undergoing ramification in the substance of the liver, convey to the capillaries 

 of that organ the blood collected in the main trunk. This blood, together with that 

 of the hepatic artery, after having served for the secretion of the bile and the 

 nourishment of the liver, is withdrawn from that organ by the hepatic veins, and 

 carried by them into the inferior vena cava. 



The portal vein or vena portae is about three inches in length. Commencing 

 behind the head of the pancreas, and just to the left of the inferior vena cava at the 

 level of the first lumbar vertebra, by the junction of the splenic and superior mesen- 

 teric veins, it passes upwards and a little to the right, behind the first part of the 



1 On the arrangement of the vesical veins and their valves, see E. Hurry Fenwick, Journ. Anat., 

 xix, 1885. 



