CIRCULATION OF THE FCETUS. 



559 



CIRCULATION OF THE FCETUS. 



In the foetus there is a direct communication between the 

 right and left auricles, by the foramen ovale in the interauricular 

 septum ; and in man the large Eustachian valve, situated at 

 the anterior border of the posterior vena cava, at its entrance 

 into the auricle, directs the blood from the posterior vena cava 

 through the foramen ovale into the left auricle ; as we have 

 before remarked, the presence of this valve in quadrupeds is very 

 doubtful. Another difference between the foetus and adult is 

 the presence of a communicating branch between the pulmonary 

 artery and posterior aorta, called the cluctus arteriosus. There 



FIG. 217. 



Foetus of the Bitch, with its membranes, a a, Uterine placenta ; b b, Foetal placenta ; c c, Chorion ; 

 d d, Arnnion seen through former ; e e, Foetus visible through its coverings. 



are two large arteries, the umbilical or hypogtutric, which arise 

 from the internal iliacs, pass along the sides of the bladder to its 

 fundus, and out of the umbilical opening, being continued along 

 the umbilical cord to the placenta ; in their course down the 

 cord they wind spirally round the umbilical vein. Finally, the 

 umbilical vein leaves the placenta, passes up the cord, and enters 

 the umbilicus, running along the floor of the abdomen, to reach 

 the liver, where it joins the vena porta. We may now proceed to 

 trace the course of the foetal circulation. 



The purified blood leaves the placenta by the umbilical vein, 

 which is formed by the junction of numerous radical branches, and 

 passes along the umbilical cord, accompanied by the two arteries ; 



