192 



TOPOGRAPHICAL ANATOJVIY OF THE 



The Fa-yrAL Circulation. 



During the period when the embryo is developing i7i utero, the 

 respiratory and digestive organs do not function. Some special and 

 temporary provision is therefore necessary whereby oxygenation of the 

 blood and a supply of nutritive material may be assured. A highly 

 vascular placenta is formed partly by the extra-embryonic portion of 

 the ovum, and partly by the mucous membrane of the uterus of the 



Liver. 

 Diaphragm. • Aorta. 



Vena cava caudalis. 



V. portae. 



Ductus arteriosus 

 Truncus brachiocephalicus. I 



Aa. umbilicales. 



Fig. 83. — Foetal circulation in the foal. 



mother. An umbilical cord connects the placenta with the umbilicus 

 of the embryo, and in the cord are contained the umbilical vessels 

 through which the blood passes from the embryo to the placenta and 

 back again. ^ 



The blood is returned from the placenta to the embryo by two 

 vessels that, in the horse, fuse into a single umbilical vein. This 

 follows the free margin of the falciform lisfament to the liver, where it 

 joins the portal vein. In the majority of mammals the termination of 

 the umbilical vein is connected with the caudal vena cava by a ductus 

 venosus, in addition to the union effected with the portal vein. In the 

 majority of mammals, therefore, some of the blood carried by the 



1 For an account of the early development of the circulation in the embryo, one 

 of the standard works on embryology should be consulted. 



