DISTRIBUTION OF VEINS—PORTAL SYSTEM. 231 
superficial veins, which, lie just beneath the skin, are capable 
of conveying at least as much more. The veins of the body 
in general unite in two large trunks, the superior and inferior 
vena cava ; which meet as they enter the right auricle of the 
heart (fig. 123). The superior vena cava is formed by,the 
union of the veins which return the blood from the neck (the 
jugulars ) with those which convey it from. the arms (the 
subclavians), as shown in fig. 122; and the inferior cava 
(y c, fig. 122) receives the blood from the trunk, the organs 
contained in the abdomen, and the lower extremities. 
267. There is, however, an important peculiarity in the 
distribution of the veins of the Intestines, which should not 
pass unnoticed. Instead of delivering their blood at once into 
the inferior vena cava, these veins unite into a trunk, called 
the Vena Portae (fig. 134), which enters the liver and subdivides 
into branches, whence a capillary network proceeds that per¬ 
meates the whole of its mass. It is from the venous blood, 
as it traverses this network, that the secretion of bile is 
formed; and the blood which is brought by the hepatic artery 
serves chiefly to nourish the liver,—no bile being formed from 
it, until it has become venous. The blood is carried-off from 
this double set of capillaries by the hepatic vein, which conveys 
it into the inferior vena cava. In Tishes, not only the blood 
of the intestines, but that of the tail and posterior part of the 
body, enters this “ portal ;; system, which is distributed to 
their kidneys as well as to their liver. Thus all the blood 
which flows through the portal system, has to go through two 
sets of capillaries, between each period of its leaving the heart 
by the aorta, and its return to it by the vena cava. 
268. We have yet to notice the lesser circulation, which 
is confined to the Lungs only. The venous blood which is 
returned to the heart by the vense caves, enters the right 
auricle, and thence passes into the right ventricle. By the 
contraction of this last cavity, it is expelled through the pul¬ 
monary artery (fig. 123), which soon divides into two main 
trunks that proceed to the right and left lungs respectively. 
The right trunk again subdivides into three principal branches, 
which are distributed to the three lobes or divisions of the 
right lung; whilst the left divides into two branches, which 
are in like manner distributed to the two lobes of the left 
lung. The capillaries, into which these branches ultimately 
