Fig. 256.—Final stage of development of portal and umbilical veins in the placental period.
Fig. 257.—Schema of relation of postcava to hepatic veins and ductus venosus.

The blood, therefore, returned to the liver by the left umbilical vein divides at the transverse fissure into three streams. Two of these pass through the connection with the portal vein and through branches developed from the hepatic part of the umbilical vein into the capillary system of the right and left lobe. The third continues through the ductus venosus to the common hepatic vein and sinus venosus ([Fig. 256]). The ductus venosus thus becomes the chief vessel returning arterialized placental blood to the heart. When the postcava develops fully the hepatic segment of this vessel also joins the terminal part of the ductus venosus ([Fig. 256]) and gradually replaces the same as the main returning venous channel, the proximal part of the ductus venosus being incorporated in the vena cava ([Fig. 257]). The postcava then receives the right hepatic veins separately, while the left hepatic veins and ductus venosus open together into the main vein. This condition obtains up to the time of birth and the consequent interruption of the placental circulation.

While at first the ductus venosus communicates throughout its entire length with the meshwork of the hepatic capillary system, a separation into two segments, i. e., ductus venosus proper and intrahepatic segment of umbilical vein, is established after the free communication with the left umbilical vein takes place. This condition is exhibited in [Fig. 258], which represents the corroded venous system of the fœtal liver, and in [Fig. 259], showing an injected liver in the fœtus at term.

Fig. 258.—Corrosion preparation of venous system of human liver in fœtus at term. (Columbia University Museum, No. 1834.)
Fig. 259.—Injected and hardened human liver from fœtus at term. (Columbia University Museum, No. 1853.)
Fig. 260.—Diagram of intrahepatic fœtal venous circulation.
Fig. 261.—Diagram illustrating the changes in the intrahepatic venous circulation resulting from the cessation of the placental circulation at birth.

It will be observed that the umbilical vein on entering the liver gives off a large branch to the left lobe, and a smaller branch on the right side to the quadrate lobe, which act as the main venæ advehentes of these portions of the liver. Arrived at the transverse fissure the umbilical vein divides into three branches, at right angles to each other. The left branch enters the left lobe, the right branch becomes directly continuous with the left main division of the portal vein, while the central branch, continuing the direction of the umbilical vein, passes dorsad, as the ductus venosus proper, to join the left hepatic vein close to its entrance into the postcava.

5. Changes Consequent upon the Establishment of Pulmonary Respiration.—After birth the umbilical vein and its continuation, the ductus venosus, become obliterated, the former constituting the round ligament of the liver, the latter the ligament of the ductus venosus, both structures imbedded in corresponding portions of the sagittal fissure on the caudal and dorsal surfaces of the adult liver ([Figs. 284] and [286]). The lateral branches of the umbilical vein, however, in its course from the ventral margin of the liver to the transverse fissure ([Fig. 258]), remain pervious and are transferred to the portal circulation.

It will be noticed, in reference to the direction of the blood current, that at birth a sudden reversal takes place in the right terminal branch of the umbilical vein at the transverse fissure ([Figs. 260] and [261]). Before birth the blood current of the umbilical vein divides into three streams, right, left and central. The latter enters the ductus venosus. The left enters the liver directly, the right traverses, from left to right, the segment between the termination of the umbilical and the bifurcation of the portal vein. This segment in the adult carries blood from right to left, as left branch of the portal vein. In the fœtus, however, the blood traverses this segment from left to right, in passing from the umbilical to the right branch of the portal vein. The blood entering the liver through the portal vein passes chiefly into the right division of that vessel ([Fig. 260]).

After birth all the venous blood entering the liver passes through the portal vein. In the right division the direction of the current is the same as in the fœtus.