518 URO-GENITAL SYSTEM. 



those relations of interchange with the blood of the mother 

 already spoken of (p. 507). We have now returned to 

 the point from which we set out, and have successively passed 

 through the various segments of the circle of the placental 

 circulation. We are now prepared to review in a brief 

 space, and present a summary of the method by which the 

 blood moves through the vessels, from the foetus to the 

 placenta, and from the placenta to thefoatus; also how this 

 placental circulation mingles with the circulation in the 

 different parts of the embryo (head, limbs, and viscera). 



Summary. The blood comes from the placenta through 

 the umbilical vein and goes to the lower surface of the liver, 

 thence it is returned into the lower vena cava by two differ- 

 ent channels ; a portion returns directly to the lower vena 

 cava through the ductus venosus of Arantius; the other 

 portion goes through the left branch of the portal vein, is 

 distributed in the left lobe of the liver, whence it flows 

 through the corresponding hepatic veins to the lower vena 

 cava ; it may be noticed that the left lobe of the liver, by 

 this arrangement, receives a mixture of intestinal venous 

 blood (portal vein) and a blood, which has been vivified by its 

 passage through the placenta (umbilical vein), whilst the 

 right lobe receives only the intestinal blood. This explains 

 the increased size and development of the left lobe ; since 

 the preponderance of the lobes is reversed in the liver of 

 adult age. 



The blood from the inferior vena cava passes into the right 

 auricle ; yet it apparently only skims through this cavity 

 without being mingled with the blood from the superior vena 

 cava. In fact (see p. 516), the blood of the inferior vena 

 cava, guided by the Eustachian valve, passes through the 

 foramen ovale into the left auricle, thence into the left ven- 

 tricle, and so into the arch of the aorta. At the last- 

 named place a small part of this blood is taken by the 

 descending portion of the aorta, where we shall presently 

 find it mingling with the blood furnished by the ductus 

 arteriosus ; the larger part of the blood received into the 

 arch of the aorta traverses the brachio-cephalic (innominate), 

 the carotid, and subclavian trunks, for the purpose of furnish- 

 ing nutrition for the head and arms. Let us not forget that 

 this blood, thus supplied to the upper extremities of the 

 embryo, is almost wholly arterial, that is to say, has been 

 vivified by the placental haBinatosis, with scarcely any venous 

 blood (only that from the inferior vena cava and the hepatifl 



