560 



EMBRYOLOGY. 



on entering the abdomen, it passes forwards along the floor of the 

 abdominal cavity, covered by peritoneum, and thus arrives at the 

 liver ; it joins the portal vein, its blood thus gaining the hepatic 

 veins, and afterwards the posterior vena cava, where it mingles with 

 the venous blood from the posterior region. The mixed blood is 

 conveyed by the posterior cava into the right auricle, and passes 

 directly through the foramen ovale into the left auricle ; from the 

 left auricle it enters the left ventricle, and by it is forced into the 

 aorta ; the major part passing into the anterior aorta, is distributed 

 to the head and fore extremities ; the remainder passes into the 

 posterior aorta. 



FIG. 218. 



Trunk of a fully-developed foetus opened from the left side to show the circulation, a, Um- 

 bilical cord, cut and ligatured ; 6 b, Umbilical vein ; c, Portal vein ligatured ; d d, Posterior vena 

 cava ; e e, Openings of hepatic veins ; /, Diaphragm cut and reflected ; g, Heart ; h, Pulmonary 

 artery ; i, Common aorta ; i' , Anterior aorta ; i" i", Posterior aorta ; k, Ductus arteriosus ; I, Left, 

 I', Eight umbilical arteries ; m, Bladder ; n, Urachus ; o o, Thymus gland. 



Venous blood is returned from the anterior region to the right 

 auricle by the anterior vena cava, and passes directly into the 

 right ventricle along with a small portion of the blood from the 

 posterior vena cava ; the right ventricle propels it through the 

 pulmonary artery, from which only a small portion passes into the 

 impervious lungs, to be returned to the left auricle by the pul- 

 monary veins, the greater part going through the ductus arteriosus 

 into the posterior aorta, where it becomes mixed with a portion 

 of the blood from the left ventricle. In the posterior aorta, it 

 flows backwards to supply the posterior extremities and abdominal 

 viscera ; the chief portion, however, on arriving at the termina- 

 tion of the posterior aorta, enters the umbilical arteries, and by 



