586 EMBRYOLOGY. 



the portal vein. Tt is to be seen in fig. 323 as an unpaired vessel 

 (pf.a). It empties into the right afferent hepatic vein, derives its- 

 roots from the region of the intestinal canal, and conveys the venous 

 blood from the latter into the right lobe of the liver. It takes its 

 origin from the two primitive vitelline veins. 



According to the account of His, the two vitelline veins fuse along 

 the tract where they run close together on the intestinal canal ; on 

 the contrary, in the region where they run to the liver and arc 

 connected with each other to form two ring-like anastomoses, each of 

 which encircles the duodenum, an unpaired trunk is formed by the 

 atrophy of the right half of the lower [posterior] ring and the left 

 half of the upper one. The portal vein thus formed therefore runs 

 first to the left and backward [dorsad] around the duodenum, and 

 then emerges on the right side of it ; it draws its blood partly from 

 the yolk-sac and partly from the intestinal canal through the vena 

 mesenterica. Afterwards the first source is exhausted with the 



regressive metamorphosis of the yolk-sac, but the other becomes more 

 and more productive with the enlargement of the intestine, the 

 pancreas, and the spleen, and during the last months of pregnancy 

 conveys a large stream of blood to the liver. 



The additional changes, which occur at birth, are easily compre- 

 hended (fig. 323). With the detachment of the after-birth the 

 placental circulation ceases. The umbilical vein (n.v) conveys no 

 more blood to the liver. That portion of its tract which extends- 

 from the umbilicus to the porta hepatis degenerates and becomes a 

 fibrous ligament (the lig. hepato-umbilicale or lig. teres hepatis), 

 Likewise the ductus Arantii (d.A) produces the well-known ligament 

 enclosed in the left sagittal fissure (lig. venosum). The right and 

 left vena3 hepaticse advehentes (ha.d, ha.s) again receive jbheir blood, 

 as in the beginning of the development, from the intestinal canal 

 through the portal vein (pf.a). 



Now that we have become acquainted with the details of the 

 morphological changes, I close this section on the vascular system 

 with a short sketch of the f<r.tal circulation of the blood. It is cha- 

 racteristic of this that no division into two separate circulations, into 

 the major or systemic and the minor or pulmonary, has yet taken 

 place ; further, that in most of the vessels neither purely arterial nor 

 purely venous blood circulates, but a mixture of the two. Purely 

 arterial blood is contained only in the umbilical veins as they come 

 from the placenta, whence we will follow the circulation. 



Having arrived at the liver, the current of the umbilical veins is 



