PLACENTAL CIRCULATION 359 



tachian valve, together with the direction of the entering 

 current, causes the blood from the ascending vena cava to 

 pass through the foramen ovale into the left auricle. 



Blood from the upper extremities (impure) enters the 

 right auricle through the descending vena cava. The Eus- 

 tachian valve and the direction of the current here again 

 cause this blood to enter the right ventricle. There is sup- 

 posed to be very little mingling of blood from the two venae 

 cavae as it passes thus through the right auricle. At the 

 same time the blood which has entered the left auricle 

 through the foramen ovale, augmented slightly by blood 

 from the ill-developed pulmonary veins, passes into the left 

 ventricle. The ventricles now contract simultaneously. 



Blood from the right ventricle (impure) passes in small 

 part through the pulmonary artery to the lungs, but chiefly 

 through a tube, the ductus arteriosus, into the descending 

 part of the aortic arch. 



Blood from the left ventricle (mixed) enters the aorta and 

 goes to the system at large. 



The vessels going to the head and upper extremities are 

 given off from the aortic arch before it is joined by the 

 ductus arteriosus. Since the ductus arteriosus contains im- 

 pure blood, the supply going to the upper extremities is 

 purer than that going to the lower. 



Of the blood which passes down the aorta a part leaves 

 by the hypogastric arteries, to go again to the placenta, while 

 the other part is distributed to the trunk and lower extremi- 

 ties. 



It thus appears that the liver is the only organ of the 

 fetus which receives pure blood, and that the head and upper 

 extremities are better provided for in this respect than are 

 the lower parts. This may account for the relatively large 

 liver of the fetus, and for the fact that the upper extremities 

 are better developed than the lower. 



The ductus arteriosus, ductus venosus, foramen ovale, 

 Eustachian valve, hypogastric (umbilical) arteries and the 



