DEVELOPMENT OF THE EMBRYO. 165 



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communicate by a transverse duct, which ultimately becomes the left 

 innominate vein. The left canal of Cuvier atrophies and becomes a fibrous 

 cord. A transverse branch now appears, which carries the blood from the 

 left cardinal vein into the right, and becomes the vena azygos minor ; the 

 right cardinal vein becomes the vena azygos major. 



Circulation of Blood in the Foetus. The blood returning from the 

 placenta, after having received oxygen, and being freed from carbonic 

 acid, is carried by the umbilical vein to the under surface of the liver ; here 

 a portion of it passes through the ductus venosus into the ascending vena 

 cava, while the remainder flows through the liver, and passes into the vena 

 cava by the hepatic veins. When the blood is emptied into the right 

 auricle, it is directed by the Eustachian valve, through the foramen ovale, 

 into the left auricle, thence into the left ventricle, and so into the aorta to 

 all parts of the system. The venous blood returning from the head and 

 upper extremities is emptied, by the superior vena cava, into the right 

 auricle, from which it passes into the right ventricle, and thence into the 

 pulmonary artery. Owing to the condition of the lung, only a small por- 

 tion flows through the pulmonary capillaries, the greater part passing 

 through the ductus arteriosus, which opens into the aorta at a point below 

 the origin of the carotid and subclavian arteries. The mixed blood now 

 passes down the aorta to supply the lower extremities, but a portion of 

 it is directed, by the hypogastric arteries, to the placenta, to be again 

 oxygenated. 



At birth, the placental circulation gives way to the circulation of the 

 adult. As soon as the child begins to breathe, the lungs expand, blood 

 flows freely through the pulmonary capillaries, and the ductus arteriosus 

 begins to contract. The foramen ovale closes about the tenth day. The 

 umbilical vein, the ductus venosus, and the hypogastric arteries become 

 impervious in several days, and ultimately form rounded cords. 



