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 

 (v 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 Portce (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 Fishes, 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 cavae, 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 



