144 ELEMENTS OF MAMMALIAN ANATOMY. 
iliacs. The union of the common iliac veins collecting the 
blood from the hind-limbs and the pelvic region, and the 
caudal vein, forms the beginning of the inferior vena cava. 
The common iliac is formed by the union of the external 
and internal iliac veins in the pelvis. The former is much 
the larger of the two. 
The portal system begins with the veins collecting the 
blood from the intestines, pancreas, spleen, and stomach, 
and terminates where the hepatic veins enter the vena cava. 
The inferior mesenteric vein collects the blood from the 
large intestine; the superior mesenteric, from the small in- 
testines; the gastrosplenic, from the spleen, stomach, and 
pancreas; the coronary, from the lesser curvature of the 
stomach; the gastro-cpiploica, from the greater curvature 
of the stomach; and the pancreatoduodenalis, from the pan- 
creas and duodenum. The last three may empty directly 
into the portal vein or into one of the three branches first 
named (Fig. 78). 
The superior vena cava extends from the union of the 
innominate veins to the right auricle. Three important veins 
empty into the superior vena cava: the azygos, collecting 
blood from the intercostal spaces; the sternal, lying on the 
visceral surface of the sternum; and the right vertebral, 
which, with its fellow, collects the blood from the deep 
muscles and spinal cord in the region of the atlas, and after 
making a strong anastomosis with the internal jugular vein, 
descends in company with the vertebral artery through the 
transverse foramina canal of the first six cervical vertebre. 
The left vertebral vein is a tributary of the left innominate 
(Fig. 78). 
The innominate vein is formed by the union of the subcla- 
vian and the external jugular. Sometimes the vertebral vein 
joins also in the union instead of emptying into the superior 
vena cava. The subclavian vein returns the blood from the 
