358 REPRODUCTION 



auricle through the descending vena cava. The Eustachian 

 valve and the direction of the current here again cause this blood 

 to enter the right ventricle. There is supposed 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 simul- 

 taneously. 



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 impure 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 extremities. 



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

 which receives pure blood, and that the head and upper extrem- 

 ities 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, Eusta- 

 chian valve, hypogastric (umbilical) arteries and the umbilical 

 vein are the organs which distinguish the placental circulation, 

 and they all partially disappear after birth, as will be imme- 

 diately seen. 



3. Adult Circulation. The circulation as it exists in the 



