THE THORAX 1009 



by the hepatic cylinders. At this stage the blood of the umbihcal veins 

 flows through the hepatic sinusoids, from which it is taken up by the venae 

 hepaticae revehentes. or hepatic veins, and these veins convey it to the sinus 

 venosus. 



At a later period the right umbihcal vein, for the most part, atrophies. A 

 portion of it, however, near the umbihcus, persists, and estabhshes a con- 

 nection with the epigastric veins of the abdominal wall. In this persistent 

 portion the blood can flow towards the umbihcus. As the right umbilical 

 vein undergoes atrophy, the left umbihcal vein becomes enlarged. Imme- 

 diately below the developing Uver it joins the upper duodenal venous ring, 

 formed by the vitelline veins. Meanwhile a vein is being developed, which 

 passes from the convexity of the upper duodenal venous ring — that is to say, 

 from the left division of the portal vein — to the right vena hepatica revehens, 

 or right hepatic vein, near the sinus venosus. This new vein constitutes the 

 ductus venosus, or duct of Arantius, and through this channel the greater 

 part of the pure placental blood passes directly to the heart, without circu- 

 lating through the liver. A certain amount of pure placental blood, however, 

 reaches the liver through the left vena hepatica advehens, which, as pre- 

 viously stated, forms a part of the left division of the portal vein. 



The left vena hepatica revehens, or left hepatic vein, loses its connection 

 with the sinus venosus, and now opens into the right vena hepatica revehens, 

 or right hepatic vein, at the level where the latter receives the ductus venosus. 

 A common venous stem is now formed, representing (i) the right hepatic 

 vein, (2) the left hepatic vein, and (3) the ductus venosus. This stem is 

 known as the vena hepatica cowmunis, and it gives rise to the terminal or 

 cardiac portion of the prerenal division of the inferior vena cava. 



The left umbihcal vein and the ductus venosus remain pervious, and are 

 important vessels, until the period of birth, being concerned in the placental 

 circulation. After the cessation of this circulation both vessels atiophy, 

 the ductus venosus entirely, and the left umbihcal vein almost entirely. 

 The obliterated ductus venosus is known as the ligamentum ductus venosi, 

 and the obliterated left umbilical vein forms the round ligament of the liver. 

 A minute portion of the lumen of the left umbilical vein remains pervious 

 within the round ligament of the hver. This pervious portion communicates 

 at the liver with the left division of the portal vein, and at the umbihcus it is 

 connected with the epigastric veins of the abdomiucd wall. It thus forms a 

 channel of communication between the left division of the portal vein and 

 the systemic veins of the anterior abdominal wall. In this pervious portion 

 the blood can flow towards the umbUicus. This anastomosis betiveen the 

 portal and systemic circulations accounts for the enlargement of the veins 

 of the anterior abdominal wall in cases of ported obstruction within the Uver. 



Summary of the Umbilical Veins. 



The umbilical veins return the blood from the placenta to the sinus venosus. 

 The two veins unite and form a single trunk within the umbUical cord. At 

 the umbihcus this trunk enters the body of the embr^-o, and immediately 

 divides into two umbihcal veins, right and left, which traverse the septxun 

 transversum and open into the sinus venosus. They soon lose their con- 

 nection with this sinus, and become broken up into sinusoids by the hepatic 

 cylinders. At a later period the right umbUiccd vein, for the most part, 

 atrophies ; but a portion of it, near the umbilicus, persists and conveys blood 

 towards the umbihcus, where it communicates with the epigastric veins of 

 the abdominal wall. The left umbilical vein now enlarges, and, immediately 

 below the developing hver, it joins the upper duodenal venous ring. Mean- 

 while a vein is being developed, which passes from the convexity of the upper 

 duodenal ring — that is to say, from the left division of the portal vein — to 

 the right hepatic vein. This new vessel is the ductus venosus, and through it 

 the greater part of the pure placental blood passes directly to the heart. The 



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