PLACENTAL CIRCULATION 357 



representing the heart has communicating with it two veins 

 at its lower extremity and two arteries at its upper. Soon 

 the tube becomes twisted upon itself so that the upper (ar- 

 terial) is thrown in front of the lower (venous). The loop 

 is V-shaped and is the outline of the future ventricles. Af- 

 terward a constriction forms the auricle. At this time the 

 heart consists of a single ventricle and a single auricle. 

 Later the ventricular and auricular septa are formed. The 

 latter appears after the former and is incomplete ; the open- 

 ing left between the auricles is the foramen ovale. 



2. Placental Circulation. As the allantois is developed 

 and the vitelline circulation is abolished, the hypogastric ar- 

 teries are given off first from the aorta, but later (with the 

 development of the vessels of the lower extremities) they 

 are pushed down, as it werej so that they take origin from 

 the internal iliacs. They pass to the umbilicus and thence to 

 the placenta by the cord. Blood is at first returned from the 

 placenta by two umbilical veins, but these soon fuse into one. 



Object of Placental Circulation. Since the activity of the 

 respiratory and alimentary tracts has not been established, 

 their functions must be performed by those of the mother 

 and the necessary materials supplied from her blood. Con- 

 sequently there must be a continual passage of fetal blood to 

 and from the placenta to discharge effete matter and to ab- 

 sorb nutriment. Certain modifications of the circulatory ap- 

 paratus, not requisite after birth, are necessary to bring this 

 about. 



Course of Fetal Circulation. The umbilical vein contain- 

 ing blood enriched with oxygen and other materials enters 

 the body at the umbilicus and passes to the under surface of 

 the liver. Hbre it divides into two branches. The larger 

 joins the portal vein and enters the liver ; the smaller is the 

 ductus venosus, which enters the ascending vena cava. 



The ascending vena cava, when it enters the right auricle, 

 therefore, contains blood from the lower extremities, blood 

 which has come from the placenta directly through the 



