422 ELEMENTARY ANATOMY. [LESS. 



The systemic arteries (excluding retia mirabilia and gill 

 structures from consideration) never, after dividing, reunite in 

 a second aggregation. 



The same is not the case with the veins, for certain of 

 them, on their way back to the heart, break up into a minute 

 network in the liver, whence they reunite and emerge in a 

 large trunk, which then unites with the rest of the systemic 

 venous system, and enters the heart. This secondary distri- 

 bution of blood in the liver (the primary distribution there, 

 being that of arterial blood by the hepatic artery) is what 

 is called the portal circulation. The veins which diverge and 

 become smaller in the liver conveying venous blood into it 

 are called portal veins. The veins which collect them- 

 selves, unite and so become larger in the liver conveying 

 venous blood out of it are called hepatic veins. 



The blood from the brain and from within the cranium 

 is collected on each side into the internal jugular vein, 

 which passes out of the skull at the foramen lacerum pos- 

 terius, and, descending the neck, joins the subclavian vein 

 (of the same side of the body), which is the one bringing 

 back the blood from the arm. 



The blood from the outside of the head, from the muscles, 

 teeth, &c., is collected on each side by the external jugular, 

 which descends the neck and also opens into the subclavian. 



A great trunk, the innominate, is thus formed (by the 

 union of the two jugulars and the subclavian) on each side 

 of the body, and the two innominate veins joining together 

 form a yet greater trunk, called the vena cava superior, which 

 opens, as has been already mentioned, into the right auricle. 



The veins of the legs collect themselves together into a 

 great vein, called the external iliac, which passes in (from 

 the limb to the abdomen) over the front brim of the pelvis, 

 and is joined by the internal iliac, bringing blood from the 

 pelvic viscera. 



This junction forms what is called the common iliac vein, 

 there being, of course, one for each side of the body. 



The two common iliacs unite and form one large and long 

 ascending trunk, called the vena cava inferior, which perforates 

 the diaphragm, and pours its blood also into the right auricle. 



On its way this trunk receives blood from a middle sacral 

 vein (corresponding with the similarly-named artery) ; from 

 the kidneys (by short, wide renal veins) ; it also receives blood 

 from certain other parts ; and, finally, it receives blood from 

 the hepatic veins, of which more must be said. 



