The Veins 327 



The allantoic veins are at first paired, but they undergo a 

 variable extent of fusion in the different species of animals ; in 

 our domestic animals they are usually united into a single vein 

 within the body of the fetus at an early date, while externally, 

 in the umbilical cord, they usually remain separate. These 

 veins, arising from the placenta, pass along the sides of the 

 allantoic stalk, or urachus, and eventually reach the sinus 

 venosus and empty into that cavity. lyater, they lose their 

 connection with the sinus venosus, the right vein becomes 

 smaller and finally disappears, while the left increases much in 

 volume and gives off some vessels as it nears the liver, which 

 enter it directly, while the main portion of the vessel joins the 

 hepatic portal vein prior to its entrance into that gland. 



At a slightly later date, when both vitelline and allantoic 

 vessels have ceased to empty into the sinus venosus, all the blood 

 from these two vessels must pass through the capillaries of the 

 liver before it can reach the heart. At this period, a communi- 

 caticfn, the ductus venosus, is established between the right 

 hepatic and portal veins, by which the blood may pass from the 

 vitelline and allantoic vessels across to the hepatic vein, without 

 passing through the capillaries of the liver. This ductus venosus 

 grows rapidly and affords a commodious and direct path, through 

 which the blood may pass at once from the placenta to the heart, 

 without going through the liver. It becomes obliterated in the 

 foal some time prior to birth so that, in this animal, all blood 

 from the allantois must pass through the liver. 



The posterior vena cava does not acquire much size until the 

 hind limbs begin to form and the iliac veins unite to constitute 

 its principal branches. It enters into the ductus venosus near its 

 termination in the hepatic vein. 



The anterior and posterior cardinal veins unite to form the 

 Cuvierian veins. The anterior cardinal vein persists in some of 

 our adult domestic animals, such as the cow, as the external 

 jugular vein. The posterior cardinal vein disappears in its 

 middle and the posterior end becomes the internal iliac vein, while 

 the anterior portion of the right posterior cardinal vein persists 

 as the vena azygos. 



The anterior vena cava represents the right anterior Cuvierian 

 vein, while the left vein disappears completely, with the possible 

 exception of the coronary sinus of the heart. 



