694 



HANDBOOK OF PHYSIOLOGY. 



In many of the lower mammals, such as the rat, the left ductns Cu- 

 vieri remains as a left superior cava. 



Meanwhile, a transverse branch carries across most of the blood of 

 the left posterior cardinal vein into the right; and by this union the 

 great azygos vein is formed. 



FIG. 480. Diagrams illustrating the development of the great veins, dc, ducts of Cuvier; / 

 jugular veins; 7i, hepatic veins; c, cardinal veins; s, subclavian vein; ji, internal jugular vein; je 

 external jugular vein; az, azygos vein; ci, inferior vena cava; r, renal veins; i7, iliac veins; hij, ny 

 pogastric veins. (Gegenbauer.) 



The upper portions of the left posterior cardinal vein remain as the 

 left superior intercostal and vena azygos minor (Fig. 480). 



CIRCULATION OF BLOOD IN THE FOETUS. 



The circulation of blood in the foatus differs considerably from that 

 of the adult. It will be well, perhaps, to begin its description by trac- 

 ing the course of the blood, which, after being carried out to the pla- 

 centa by the two umbilical arteries, has returned, cleansed and replen- 

 ished, to the foetus by the umbilical vein. 



It is at first conveyed to the under surface of the liver, and there the 

 stream is divided, a part of the blood passing straight on to the inferior 

 vena cava, through a venous canal called the ductus venosus, while the 

 remainder passes into the portal vein, and reaches the inferior vena 

 cava only after circulating through the liver. Whether, however, by the 

 direct route through the ductus venosus or by the roundabout way 

 through the liver, all the blood which is returned from the placenta 

 by the umbilical vein reaches the inferior vena cava at last, and is car- 

 ried by it to the right auricle of the heart, into which cavity is also 

 pouring the blood that has circulated in the head and neck and arms, 



