DEVELOPMENT OF THE VASCULAR SYSTEM. 695 



are, first, the liver and the cluctus venosus, and secondly, the heart 

 and ductus arteriosus. 



The Hepatic Circulation and Ductus Venosus. The liver appears 

 at a very early period, in the upper part of the abdomen, as a mass of 

 glandular and vascular tissue, developed around 

 the upper portion of the omphalo-mesenteric 

 vein, just below its termination in the heart 

 (Fig. 242). As soon as the organ has attained 

 a considerable size, the omphalo-mesenteric 

 vein ( t ) breaks up in its interior into a capil- 

 lary plexus, the vessels of which reunite into 

 a venous trunk, conveying the blood toward 

 the heart. The omphalo-mesenteric vein below 

 the liver then becomes the portal vein ; while 



above that organ it receives the name of the E" 1 * form of the HEPATIC 



__, .. ,. , CIRCULATION. 1. Omphalo- 



hepatic vein (,). The liver, accordingly, is at mese nteric vein. 2. Hepatic 

 this time supplied with blood entirely by the vein - 3 - Heart - The dotted 



. . ,. . .,. , . , line shows the situation of 



portal vein, coming from the umbilical vesicle the future umbilical vein. 

 and the intestine ; and all the blood derived 



from this source passes through the hepatic circulation before reaching 

 the heart. 



But soon afterward the allantois makes its appearance, and becomes 

 developed into the placenta ; and the umbilical vein returning from it 

 joins the omphalo-mesenteric vein, and takes 

 part in the formation of the hepatic capillary FIG. 243. 



plexus. Since the umbilical vesicle, however, 

 becomes atrophied and the intestine remains 

 inactive, while the placenta increases in size 

 and importance, a period arrives when the liver 



receives more blood by the umbilical vein than 

 by the portal vein (Fig. 243). The umbilical 

 vein then passes into the liver at the longitu- 

 dinal fissure, and ramifies throughout the left 

 lobe of the organ. To the right it sends a large HEPATIC ClRCULATION farther 

 branch of communication, which opens into the advanced. i. Portal vein. 2. 

 portal vein, and partially provides for the cir- m ' ll voin ' 3 " Hepatic 

 culation in the right lobe. The liver is thus 



supplied with blood from two sources, the most abundant of which is 

 the umbilical vein ; while all the blood which enters it circulates, as 

 before, through its capillary vessels. 



But the liver at this time is much larger, in proportion to the other 

 organs, than at a later period. In the foetal pig, when very young, it 

 amounts to nearly twelve per cent, of the whole body ; while before 

 birth it diminishes to seven, six, and even three or four per cent. In 

 the latter part of foetal life, therefore, its capillary circulation becomes 

 insufficient to accommodate all the blood returning from the placenta ; 

 and a vascular canal is formed in its interior, by which a portion of the 



