226 ANATOMY FOR NURSES [Chap. XII 



conveyed into the inferior vena cava partly through the Hver 

 but chiefly through the " ductus venosus," which connects these 

 two vessels. From the inferior vena cava it enters the right 

 auricle, passes through the foramen ovale into the left auricle, 

 thence into the left ventricle, and out through the aorta, which 

 distributes it principally to the upper extremities. The blood 

 from the head and upper extremities returns by the superior 

 vena cava to the right auricle, then passes into the right ventricle, 

 and out through the pulmonary artery to the lungs. As the 

 lungs in the foetus are solid, they require very little blood (only 

 for nutrition), and the greater part of the blood passes through 

 the ductus arteriosus into the descending aorta, where, mixing 

 with the blood delivered to the aorta by the left ventricle, it 

 descends to supply the lower extremities of the foetus. The 

 chief portion of this blood is carried to the placenta by the 

 two umbilical arteries, but a small amount passes back into the 

 ascending vena cava and mixes with the blood from the placenta. 

 From this description of the foetal circulation, it will be seen : — 



1 . That the placenta serves the double purpose of a respiratory 

 and nutritive organ, receiving the venous blood from the foetus, 

 and returning it again charged with oxygen and additional nutri- 

 tive material. 



2. That the liver receives pure blood directly from the placenta ; 

 hence the large size of this organ at birth. 



3. That the blood from the placenta passes almost directly 

 into the arch of the aorta, and is distributed by its branches to 

 the head and upper extremities ; hence the large size and perfect 

 development of these parts at birth. 



4. That the blood in the descending aorta is chiefly derived 

 from that which has already circulated in the upper extremities, 

 and, mixed with only a small quantity from the left ventricle, 

 is distributed to the lower extremities ; hence the small size and 

 imperfect development of these parts at birth. 



Changes in the vascular system at birth. — From the foregoing 

 description it is obvious that at birth very important changes must 

 take place : — 



1. The blood clots in the umbilical vein, between the usual 

 ligature and the liver, also in the ductus venosus. The blood clot 

 becomes organized and these two vessels become obliterated. 



