676 DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



Fig. 263. 



HEPATIC CIRCULATION 

 farther advanced. 1. Portal vein. 

 2. Umbilical vein, 

 vein. 



the intestine also remains inactive, while the placenta increases in 

 size and in functional importance, a time soon arrives when the 



liver receives more blood by the umbilical 

 vein than by the portal vein. (Fig. 263.) 

 The umbilical vein then passes into the 

 liver at the longitudinal fissure, and sup- 

 plies the left lobe entirely with its own 

 branches. To the right it sends off a large 

 branch of communication, which opens in- 

 to 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 

 3. Hepatic w hich is the umbilical vein ; and all the 

 blood entering the liver circulates, as be- 

 fore, through its capillary vessels. 



But we have already seen that the liver is much larger, in pro- 

 portion to the entire 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 ; but be- 

 fore birth it diminishes to seven, six, 

 and even three or four per cent. For 

 some time, therefore, previous to 

 birth, there is much more blood re- 

 turned from the placenta than is re- 

 quired 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 

 This duct is 



Fig. 264. 



HEPATIC CIRCULATION during lat- 

 ter part of foetal life. 1. Portal vein. 2. 



Umbilical vein. 3. Left branch of umbili- Jl6 P atK Capillaries, 

 cal vein. 4. Right branch of umbilical Called the DuctllS VeUOSUS. 

 vein. 5. Ductus venosus. 6. Hepatic 

 vein. 



The ductus venosus is formed by a 

 gradual dilatation of one of the he- 

 patic capillaries at (5) (Fig. 264), which, enlarging excessively, be- 

 comes at last converted into a wide canal, or branch of communi- 

 cation, passing directly from the umbilical vein below to the hepatic 

 vein above. The circulation through the liver, thus established, is 



