THE VENOUS SYSTEM. 663 



The vitelline vein is the first part of this system established, and divides 

 near the heart into two veins bringing back the blood from the yolk-sack 

 (umbilical vesicle). The right vein soon however aborts. 



The allantoic (anterior abdominal) veins are originally paired. They 

 are developed very early, and at first course along the still widely open 

 somatic walls of the body, and fall into the single vitelline trunk in front. 

 The right allantoic vein disappears before long, and the common trunk 

 formed by the junction of the vitelline and allantoic veins becomes con- 

 siderably elongated. This trunk is soon enveloped by the liver. 



The succeeding changes have been somewhat differently described by 

 Kolliker and Rathke. According to the former the common trunk of the 

 allantoic and vitelline veins in its passage through the liver gives off 

 branches to the liver, and also receives branches from this organ near its 

 anterior exit. The main trunk is however never completely aborted, as in 

 the embryos of other types, but remains as the ductusvenosusArantii. 



With the development of the placenta the allantoic vein becomes the 

 main source of the ductus venosus, and the vitelline or portal vein, as it may 

 perhaps be now conveniently called, ceases to join it directly, but falls into 

 one of its branches in the liver. 



The vena cava inferior joins the continuation of the ductus venosus in 

 front of the liver, and, as it becomes more important, it receives directly 

 the hepatic veins which originally brought back blood into the ductus 

 venosus. The ductus venosus becomes moreover merely a small branch of 

 the vena cava. 



At the close of foetal life the allantoic vein becomes obliterated up to its 

 place of entrance into the liver ; the ductus venosus becomes a solid cord 

 the so-called round ligament and the whole of the venous blood is brought 

 to the liver by the portal vein 1 . 



Owing to the allantoic (anterior abdominal) vein having merely a fcetal 

 existence an anastomosis between the iliac veins and the portal system by 

 means of the anterior abdominal vein is not established. 



BIBLIOGRAPHY of the Venous System. 



(498) J.Marshall. "On the development of the great anterior veins." Phil. 

 Trans., 1859. 



(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Sauge- 

 thieren." MeckeVs Archiv, 1830. 



(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbel- 

 thiere." Bericht. iib. d. nalitrh. Seminar, d. Univ. Konigsberg, 1838. 



Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke 

 (Nos. 299, 300, and 301). 



1 According to Rathke the original trunk connecting the allantoic vein directly 

 with the heart through the liver is aborted, and the ductus venosus Arantii is a 

 secondary connection established in the latter part of foetal life. 



