RETIA MIRABILIA 431 



where it passes through the liver, gradually disappears. In the 

 meantime, the cceliac and mesenteric veins have appeared, 

 and all the blood from them, as well as from the vitelline veins, 

 now passes through a common trunk, the hepatic portal vein, into 

 the capillaries of the liver, whence it reaches the sinus venosus 

 through the hepatic veins. The portions of the vitelline veins 

 posterior to the liver also gradually disappear as the yolk-sac 

 becomes reduced. 



In addition to these vessels, the umbilical vein must also be 

 mentioned. This vessel also is originally paired, and corresponds 

 phylogenetically to the lateral veins of Elasmobranchs and to the 

 abdominal or epigastric vein of Ceratodus and Amphibians. It 

 is situated originally in the body-walls, and comes into relation 

 with the allantois (p. 9), opening primarily into the sinus 

 venosus and eventually into the postcaval : as the allantois 

 increases in size it brings back the oxygenated blood from this 

 organ (i.e., from the placenta in the higher Mammalia) to the 

 embryo. The right umbilical vein, however, is early obliterated, 

 and the left comes into connection with the capillaries of the 

 liver, its main stem in this region disappearing (Fig. 331, B) ; 

 thus the blood from the allantois has to pass through the capil- 

 laries of the liver before reaching the heart. In the course of 

 development, however, a direct communication is formed between 

 the left umbilical vein and the revehent branches of the fused 

 vitelline veins, and this trunk is known as the ductus venosus 

 (Fig. 331, c) : the point at which it opens corresponds to that from 

 which the postcaval has in the meantime arisen, the hepatic veins 

 now appearing as factors of the latter. On the cessation of the 

 allantoic (or placental) circulation, the ductus venosus becomes 

 degenerated into a fibrous cord, so that all the portal blood has to 

 pass through the capillaries of the liver. 



The intra-abdominal portion of the umbilical vein persists 

 throughout life as the epigastric vein in Reptiles and in Echidna, 

 but disappears in Birds and in other Mammals. 1 



Retia Mirabilia. 



By this term is understood the sudden breaking- up of an arte- 

 rial or venous vessel into a cluster of fine branches, which, by 

 anastomosing with one another, give rise to a capillary network ; 

 the elements of this network may again unite to form a single 

 vessel. The former condition may be described as a unipolar, the 

 latter as a bipolar rete mirabile. If it is made up of arteries or of 



1 The mode of development of the veins of the extremities is essentially 

 similar in all the Amniota, and at first resembles that occurring in Urodela, 

 though later on considerable differences arb seen, more especially as regards the 

 veins of the digits. 



