Development of the Fourth Day 261 



entire supply of blood to the liver is derived 

 from the mesenteric vein, by which it is brought 

 from the hinder part of the digestive tract ; the 

 mesenteric vein may now be called the hepatic 

 portal vein. 



The closure of the ductus venosus, by which 

 all of the blood brought to the liver by the 

 portal vein is compelled to pass through the 

 hepatic capillaries before reaching the heart, 

 has already been mentioned. 



It is not until some time after hatching that 

 the complete closure of the foramen ovale (the 

 opening between the right and left auricles) 

 takes place. By the closure of this opening, 

 all of the blood brought to the heart by all 

 three vense cavse is emptied into the right 

 auricle ; and when that auricle contracts, all of 

 this blood is forced into the right ventricle, 

 none of it finding its way directly into the left 

 auricle. As a result of this, also, all of the 

 blood that gets Into the left auricle, and, con- 

 sequently, into the left ventricle, comes from 

 the lungs by the pulmonary veins. Thus are 

 the arterial and venous streams of blood com- 

 pletely separated, and the double circulation 

 is established. 



Before passing to the development of the 



