THE MAMMALIAN VENA CAVA POSTERIOR 41 



grammatically represented than in figure 54. In this instance 

 the right and the left caval veins in the lumbar region ( supra - 

 cardinals, Type BC), incorrectly designated by Lauber as 

 posterior cardinal veins, are directly continuous in the thor- 

 acic region with a single azygos vein, which, in the middle 

 thoracic region, swings, dorsal to the aorta, from the left to 

 the right side, and opens into the vena cava superior. This 

 single azygos vein shows indications of having been formed 

 by a fusion, dorsal to the aorta, between the supracardinal 

 veins of opposite, sides. In addition to the supracardinal line 

 as a pathway for the return of blood to the heart from the 

 iliac region, there is present also a typical vena cava inferior 

 (V. cava inf., fig. 55), which joins the supracardinal system 

 at the renal level and, after passing through the liver, opens 

 into the right auricle. 



We now pass to a consideration of those cases which fall 

 within group II. These are illustrated by figures 56, 57, and 

 58. Here the supracardinal system of veins has persisted in 

 the adult and constitutes the sole and only direct main path- 

 way by which blood is returned to the heart from the region 

 of the body drained by the iliac veins. In all variants which 

 fall under group II, the intrahepatic venous circulation, 

 formed by the advehent portal and the revehent hepatic 

 channels, is entirely independent of the systemic veins (supra- 

 cardinals). In figure 58 are shown the portal vein enter- 

 ing the liver and the vena hepatica communis emerging from 

 the liver and extending to the right auricle. An ontogenetic 

 interpretation of this form of variant is relatively simple. 

 During the course of normal development, a connection is 

 established between the hepatic circulation and the systemic 

 veins (right subcardinal), at a point designated the lie pat o- 

 subcardinal junction (McClure, '06), so that blood is directed 

 from the systemic veins to the heart through the liver. (See 

 Huntington and McClure, '20, figs. 1 and 2.) Should this con- 

 nection not be established, the only available pathway from 

 the posterior region of the body to the heart would be by the 

 supracardinal system of veins. If that portion of the vena 



