634 THE VEINS 



ut' the left renal vein may ojien. (7) It may receive .several accessory renal veins ; as many as 

 seven on each side have been met with. (8) The lumbar veins may enter it on one or both sides 

 as a common trunk. 



(8) THE COMMON ILIAC VEINS 



The common iliac veins :ire formed opposite the sacro-iliac synehondrosis by 

 tlie eontluenee oi the external iliac and internal iliac vein.<. They converge as they 

 ascend, and unite opposite the upper border of the tilth lumbar verteljra to form 

 the vena cava inferior a little to the right of the median line. 



The right vein, shorter and more vertical in direction than the left, passes 

 obliquely liehind the right common iliac artery to its outer side, where it is joined 

 by the left common iliac vein. 



The left vein lies to the inner side of the left common iliac artery, and, after 

 crossing in front of the promontory of the sacrum and fifth lumlxir vertebra below 

 the bifurcation of the aorta, passes beneath the right comnKju iliac artery to join 

 the right vein and form the inferior vena cava. The left vein may contain an 

 imperfect valve. 



Tributaries, — Tlie ilio-lumbar veins may enter the lower })art of the common 

 iliac, or open into the internal iliac vein. The left vein receives the middle sacral 

 veins. 



(a) The ilio-lumbar veins follow the course of the ilio-lumbar artery, and end 

 either in the connnon iliac or in the internal iliac vein. 



(h) The middle sacral veins ascend on either side of the middle sacral artery 

 in front of the sacrum, to o})cn usually by a single trunk into the left common iliac 

 vein. They connnunicate with the lateral sacral veins, forming the so-called pre- 

 sacral plexus. Below, the middle sacral veins communicate with the ha?morrhoidal 

 veins. 



CJdef Variations in the Common Iliac Veins 



(1) Either common iliac vein maybe double, or double only for a portion of its extent. 

 (2) One may be absent, — the external and internal iliac veins joining the opposite common iliac to 

 form the vena cava. (3) The right and left internal iliac veins may unite and open as a common 

 trunk at the confluence of tlie right and left external iliac veins to form the vena cava. (4) The 

 middle sacral ti-unk vein may divide, and one branch open into the right, and the other into the 

 left common iliac vein. 



THE PORTAL VEIN AND ITS TRIBUTARIES 



The veins corresponding to the inferior mesenteric, the superior mesenteric, and 

 the branches of the cn^liac artery, with the exception of 'the terminal branches of 

 the hepatic artery, do not join the inferior vena cava direct, but unite to form a 

 common trunk — the portal vein. 



This vein enters the liver, and breaks up in its substance into cajjillaries like an 

 artery, from which the lilood is again ultimately collected by the hepatic veins, 

 and carried by them into the inferior vena cava. The terminal branches of the 

 hepatic artery also break up in the liver into capillaries, and from them the blood 

 likewise finds its way finally into the hepatic veins, and thence into the inferior 

 vena cava. Thus the arterial blood, leaving the aorta for the su]>ply of the stom- 

 ach, the intestines, the pancreas (the so-called chylopoetic viscera), and the spleen, 

 passes, before it reaches the vena cava, through two sets of ca])illaries: viz. the 

 cajjillaries of the viscera and the capillaries of the liver. Hence the portal system 

 of veins may be said to terminate in capillaries at each end; to begin, like other 

 veins, in capillaries in the viscera; but, unlike other veins, to end in ca})illarieslike 

 an artery, instead of in a larger and larger vein till the auricle is reached. The 

 portal vein and its tributaries have no valves. 



The portal vein is a thick trunk about three inches in length (7-8 cm.). It 

 is formed Iwhind the pancreas, opposite the right side of the body of the second 

 lumbar vertebra, by the union of the superior mesenteric with the splenic vein. 

 After passing behind the first part of the duodenum, and then between the layers 



