DEVELOPMENT OF THE EMBRYO. 



[Fig. 288. 



.Ul *l I 11 ' 



721 



U U 



The Fcetal Circulation ; 1, the umbilical cord, consisting of the umbilical vein and two umbilical 

 arteries; proceeding from the placenta (2) ; 3, the umbilical vein dividing into three branches; two (4, 4) 

 to be distributed to the liver; and one (5), the ductus venosus, which eaters the inferior vena cava (G); 

 7, the portal vein, returning the blood from the intestines, and uniting with the right hepatic branch; 8, 

 the right auricle; the course of the blood is denoted by the arrow, proceeding from 8 to 9, the left auricle; 

 10, the left ventricle ; the blood following the arrow to the arch of the aorta (tl), to be distributed through 

 the branches given off by the arch to the head and upper extremities. The arrows, 12 and 13, represent 

 the return of the blood from the head and upper extremities through the jugular and subclavian veins, 

 to the superior vena cava (14), to the right auricle (8), and in the course of the arrow through the right 

 ventricle (15), to the pulmonary artery (16) ; 17, the ductus arteriosus, which appears to be a proper con- 

 tinuation of the pulmonary artery the offsets at each side are the right and left pulmonary artery cut 

 off; these are of extremely small size as compared with the ductus arleriosus. The ductus arteriosus 

 joins the descending aorta (IS, 18), which divides into the common iliacs, and these into the internal 

 iliacs, which become the umbilical arteries (19), and return the blood along the umbilical cord to the 

 placenta; while the other divisions, the external iliacs (20), are continued into the lower extremities. 

 The arrows at the termination of these vessels mark the return of the venous blood by the veins to the 

 inferior cava.] 



changed by its diversion into the Lungs ; which takes place immediately on 

 the first inspiration. The Ductus Venosus and Ductus Arteriosus soon shrivel 

 into ligaments ; the Foramen Ovale becomes closed by its valve ; and the 

 circulation, which was before carried on upon the plan of that of the higher 

 Reptiles, now becomes that of the complete Bird or Mammal. It is by no 

 means unfrequent, however, for some arrest of development to prevent the 

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