323 THE CIRCULATORY SYSTEM. 



branches unite and the common trunk joins the superior mesen- 

 teric vein (b) to form the portal vein as above described. 



5. V. mesenterica superior {b). — The superior mesenteric 

 vein receives the blood from the small and large intestines. 

 It is formed by numerous branches which follow and have the 

 same name as the corresponding arteries. It receives also the 

 inferior mesenteric {j) from the large intestine. The numer- 

 ous branches unite, and the main trunk thus formed joins the 

 gastrosplenic {c), and the two united become the portal 

 vein {a). 



Vena iLiACA communis. Common Iliac Vein (Fig. 

 126, 7i"; Fig. 127, page 310). 



The common iliac veins (Fig. 126, tv) are the two large 

 vessels which unite in the sacral region to form the inferior 

 vena cava {a). Each is a large vessel, usually four or five 

 centimeters in length, which is formed by the junction of the 

 large external iliac vein with the smaller hypogastric vein 

 [c). The left common iliac usually receives the V. sacralis 

 media {o) ; other lateral branches are as a rule not received by 

 the common iliac veins. 



There is much variation as to the formation of the common 

 iliac veins and their union to form the vena cava. On this sub- 

 ject the paper by McClure, in the American Naturalist, vol. 

 XXXIV. pp. 135-198 (March, 1900), may be consulted with 

 profit. The more typical variations may be classified as fol- 

 lows: (i) The two common iliac veins may be longer than 

 usual, so that the vena cava is formed farther craniad than 

 usual. (2) There may be in the abdominal region two sep- 

 arate veins (venae cardinales) representing the inferior vena 

 cava, these uniting in the region of the kidneys. (3) The 

 vena sacralis media may enter into the left common iliac (the 

 usual condition); or the right common iliac (as in Fig. 126); 

 or it may fork, one branch passing to the left, the other to the 

 right common iliac. 



Branches of the common iliac veins: 



I. V. sacralis media (Fig. 126, o). — This follows the 

 course of the corresponding artery and usually enters the left 

 common iliac. 



