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TEXT-BOOK OF PHYSIOLOGY. 



blood-vessels of the embryo, the umbilical arteries and vein have come into 

 histologic and physiologic relations with the uterine blood-vessels, the 

 nutritive materials and the oxygen are derived entirely from the maternal 

 blood-stream which at the same time receives carbon dioxid and perhaps 

 other waste products from the fetal blood-stream. The placenta thus 

 serves as a digestion and respiratory organ. The blood having undergone 

 these changes now leaves the placenta and returns to the fetus by the um- 

 bilical vein.^This blood is relatively rich in nutritive material and of a 



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FIG. 353. THE FETAL CIRCULATION, ao. Aorta, a.pu. Pulmonary artery, au. Umbilical 

 artery, da. Ductus arteriosus. dv. Ductus venosus. int. Intestine, vci and vcs. Inferior and 

 superior venae cavae. vh. Hepatic vein. vp. Venaporte v. pu. Pulmonary vein. vu. Umbilical 

 vein. (From Kollmann.) 



scarlet red color by reason of the presence of an increased amount of oxygen. 

 As it passes into the abdominal cavity a portion, about one-half, of the blood 

 is directed by the ductus venosus into the inferior vena cava, while the re- 

 mainder is emptied into the portal vein, by which it is distributed to the 

 liver and from which it emerges by the hepatic veins and is poured into the 

 inferior vena cava. The blood in the vena cava is thus a mixture of venous 

 blood from the lower extremities and liver, and oxygenated blood from the 

 placenta. After its discharge into the right auricle the blood is directed by 

 a fold of the lining membrane, the Eustachian valve, through an opening in 



