138 ANATOMY FOR NURSES. [CHAP. XL 



the placenta along the umbilical cord by the umbilical vein. 

 Entering the foetus at the umbilicus the blood passes upwards 

 to the liver and is conveyed into the inferior vena cava in two 

 different ways. The larger quantity first enters the liver, and 

 alone, or in conjunction with the blood from the portal vein, 

 ramifies through the liver before entering the inferior vena 

 cava by means of the hepatic veins. The smaller quantity of 

 blood passes directly from the umbilical vein into the inferior 

 vena cava by a tube called the ductus venosus. 



In the inferior vena cava the blood from the placenta becomes 

 mixed with the blood returning from the lower extremities of 

 the foetus. It enters the right auricle and guided by a valve, 

 the Eustachian valve, passes through the foramen ovale into the 

 left auricle. In the left auricle it mixes with a small quan- 

 tity of blood returned from the lungs by the pulmonary veins. 

 From the left auricle the blood passes into the left ventricle, 

 and is distributed by the aorta almost entirely to the upper 

 extremities. Returned from the upper extremities by the su- 

 perior vena cava the blood enters the right auricle and, passing 

 over the Eustachian valve, descends into the right ventricle, 

 and from the right ventricle into the pulmonary artery. As 

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

 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, however, being carried back to the placenta by 

 the two umbilical arteries. 



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



1. That the placenta serves the double purpose of a respi- 

 ratory and nutritive organ, receiving the venous blood from 

 the foetus, and returning it again charged with oxygen and 

 additional nutritive material. 



2. That the greater part of the blood traverses the liver 

 before entering the inferior vena cava ; 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 per- 

 fect development of those parts at birth. 



