212 ANATOMY AND PHYSIOLOGY 



the foramen ovate into the left atrium; from there it passes into 

 the left ventriculum and through the aorta, to be distributed. 



The greater portion of this current goes to the head and upper 

 extremities, from which it returns to the right atrium again and 

 passes down into the right ventriculum; thence into the pulmonary 

 artery, but not to the lungs (except a very small portion); it is 

 delivered instead by the ductus arteriosus to the aorta (at a point 

 where the arch begins to descend, and joins the small current 

 already there, to supply the trunk and lower extremities. 



The greater portion of this blood leaves the fetus before the 

 lower extremities are reached, by way of the two umbilical arteries, 

 and returns to the placenta for re-oxygenation ; while that which 

 does go to the lower extremities is later returned to the inferior 

 vena cava to be again mixed with blood from the umbilical vein, 

 on its way to the fetal heart. 



The external iliac supplies the lower extremities before and 

 after birth. 



Notes. The liver is the only organ to receive blood just as it comes from 

 the mother; the baby's liver is very large. The head and upper extremities 

 are next supplied, although with a slight admixture of venous blood (which 

 came through the inferior vena cava); they are well developed. The pelvis 

 and lower extremities receive but a small supply of venous with a slight 

 admixture of arterial blood; they are not so well developed. 



The placenta. The placenta is formed in a portion of the lining 

 membrane of the uterus, by an intricate arrangement of the uterine 

 vessels of the mother with the umbilical vessels of the fetus. It is 

 here that the umbilical arteries coming from the fetus end; and 

 the umbilical vein going to the fetus arises. Here also the inter- 

 change of gases and of waste and nutritive matter between fetal 

 and maternal blood is carried on, in blood spaces of the placenta. 



The umbilical cord connects the placenta and the fetus. It 

 comprises the two arteries and the one vein, protected by a gelat- 

 inous substance or " Wharton's jelly," in which they are embedded. 



