326 Veterhiary Obstetrics 



The Veins. 



When the mammalian embryo is about three weeks old, there are 

 three pairs of veins : the Cuvierian, the vitelline and the allantoic. 

 The first of these return the blood from the embryo itself and 

 are formed by the union of the cardinal veins. The vitelline 

 veins are formed in the walls of the yolk-sac and carry blood 

 from it along the vitelline stalk to the heart, while the third 

 pair return blood from the placenta along the stalk of the allantois. 



The vitelline veins df mammals are naturally small, since the 

 vitellus, or yolk, contains essentially no nutriment and, hence, 

 affords small opportunity for any important functions in these 

 veins, so long as they merely carry blood from the yolk-sac to 

 the heart. They are situated in the mesoblast of the splanchno- 

 pleure, enter the embryo at the umbilicus and, passing forward 

 along the sides of the alimentary canal, empty into the sinus 

 venosus. 



In growing out from the intestine, the liver surrounds the vitel- 

 line veins before they empty into the sinus venosus. Here theveins 

 break up into a series of capillaries, converging later to form the 

 efferent hepatic vessels. In this way, the capillary system of the 

 liver is formed and we have a vein dividing up into capillaries in 

 the same way as arteries usually do, and converging again to form 

 a second venous trunk. At about the same period that the vitelline 

 veins are breaking up to constitute the functional circulation of 

 the liver, they become connected with each other just before 

 their entrance into that gland, by three communicating branches, 

 two of which pass beneath the duodenum, and one over it, 

 and thus form venous rings surrounding the intestine. From 

 the anterior ring, there arise veins which carry the blood into 

 the liver. The right and left vitelline veins soon unite to form 

 a single trunk, which, a little later, is joined by veins returning 

 blood from the intestine, after which it is known as the hepatic 

 portal vein. 



The veins which previously established the communication 

 between the right and left vitelline veins disappear in part in 

 such a way that the remaining portions, which constitute later 

 the direct trunk, are twisted about the duodenum in a manner 

 similar to that seen in adult life. 



