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TEXT-BOOK OF PHYSIOLOGY. 



almost impervious condition of the pulmonary capillaries. On re- 

 turning from the placenta, the blood in the umbilical vein is relatively 

 rich in nutritive material and scarlet red in color from the presence 

 of oxygen. As it passes into the abdominal cavity a portion of the 

 blood is directed by the ductus venosus into the vena cava, while 

 another portion is emptied into the branches of the portal vein, by 

 which it is distributed to the liver and from which it emerges by 

 the hepatic veins and poured into the vena cava. The blood in the 

 vena cava is thus a mixture of venous blood from the lower extremi- 

 ties and liver, and oxygenated blood from the placenta. After its 

 discharge into the right auricle the blood is directed by a fold of 



Chorionic villi. 



Intervillous spaces. 



Muscularis. "j 



FIG. 333. DIAGRAM or HUMAN PLACENTA AT THE CLOSE OF PREGNANCY. 



(Schaper.) 



the lining membrane, the Eustachian valve, through an opening in 

 the interauricular septum, the foramen ovale, into the left auricle. 

 It then flows through the auriculo-ventricular opening into the left 

 ventricle, thence into the aorta, and by its branches is distributed 

 to all parts of the body. 



The blood from the head and upper extremities is emptied by 

 the superior vena cava into the right auricle, but as it passes in 

 front of the Eustachian valve, it flows directly into the right ventricle 

 and then into the pulmonary artery. On account of the unexpanded 

 condition of the lungs and the almost impervious condition of the 

 pulmonary capillaries, but a small portion of the blood passes 

 through them, while the larger portion by far passes into the aorta 



