800 DEVELOPMENT OF THE VASCULAR SYSTEM. 



below the liver then becomes the portal vein ; while above the liver, and 

 between that organ and the heart, it receives the name of the hepatic 

 vein (2). The liver, accordingly, is at this time supplied with blood 



entirely by the portal vein, coming from the 

 Fig. 306. umbilical vesicle and the intestine ; and all 



the blood derived from this source passes 

 through the hepatic circulation before reach- 

 ing the venous extremity of the heart. 



But soon afterward the allantois makes 

 its appearance, and becomes developed into 

 the placenta; and the umbilical vein re- 

 turning from it joins the omphalo-mesenteric 

 vein in the substance of the liver, and 

 takes part in the formation of the hepatic 



HEPATIC Cuter NATION 



farther advanced. - 1. Portal capillary plexus. Since the umbilical vesicle, 

 vein. 2 umbilical vein. 3. He- however, becomes atrophied, while the intes- 



patic vein. 



tine remains inactive, at the same time that 



the placenta increases in size and in functional importance, a period 

 arrives when the liver receives more blood by the umbilical vein than by 

 the portal vein. (Fig. 306.) The umbilical vein then passes into the 

 liver at the longitudinal fissure, and supplies the left lobe entirely with 

 its own branches. To the right it sends off a large branch of communi- 

 cation, which opens into the portal vein, and partially supplies the right 

 lobe with umbilical blood. The liver is thus supplied with blood from 

 two different sources, the most abundant of which is the umbilical vein ; 

 and all the blood entering the liver circulates, as before, through its 

 capillary vessels. 



But the liver is much larger, in proportion to the entiro body, at an 

 early period of foetal life than in the later months. In the foetal pig, 

 when very young, it amounts to nearly twelve per cent, of the weight 

 of the whole body; while before birth it diminishes to seven, six, and 

 even three or four per cent. For some time, therefore, during the 

 latter part of foetal life, much more blood returns from the placenta 

 than is required for the capillary circulation of the liver. Accordingly, 

 a vascular duct or canal is formed in its interior, by which a portion of 

 the placental blood is carried directly through the organ, and conveyed 

 to the heart without having passed through the hepatic capillaries. This 

 canal is the Ductus venosus. 



The ductus venosus is formed by a gradual dilatation of one of the 

 hepatic capillaries (at 5 Fig. 307), which, enlarging excessively, be- 

 comes converted into a wide branch of communication, passing from the 

 umbilical vein below to the hepatic vein above. The circulation through 

 the liver, at this period, is as follows : A certain quantity of venous blood 

 still enters through the portal vein (i), and circulates in a part of the 

 capillary system of the right lobe. The umbilical vein (2), bringing a 

 much larger quantity of blood, enters the liver a little to the left, and 

 the blood which it contains divides into three principal streams. One of 



