328 THE CIRCULATORY SYSTEM. 
branches unite and the common trunk joins the superior mesen- 
teric vein (4) to form the portal vein as above described. 
5. V. mesenterica superior (4)}.—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 (7) from the large intestine. The numer- 
ous branches unite, and the main trunk thus formed joins the 
gastrosplenic (¢), and the two united become the portal 
vein (a). 
VENA ILIACA COMMUNIS. COMMON ILIAC VEIN (Fig. 
126, w; Fig. 127, page 310). 
The common iliac veins (Fig. 126, w) are the two large 
vessels which unite in the sacral region to form the inferior 
vena cava (@). 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 
(z). The left common iliac usually receives the V. sacralis 
media (0); 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 Amertcan Naturalist, vol. 
XXXIV. pp. 135-198 (March, 1900), may be consulted with 
profit. The more typical variations may be classified as fol- 
lows: (1) 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 (vene 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: 
1. V. sacralis media (Fig. 126, 0).—This follows the 
course of the corresponding artery and usually enters the left 
common iliac. 
