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HANDBOOK OF PHYSIOLOGY. 



the thorax, causing the ducts of Cuvier to become oblique instead of 

 transverse. As the fore-limbs develop, the subclavian veins are formed. 

 A transverse communicating trunk now unites the two ducts of 

 Cuvier, and gradually increases, while the left duct of Cuvier becomes 

 almost entirely obliterated (all its blood passing by the communicating 

 trunk to the right side) (fig. 499, C.D.). The right duct of Cuvier 

 remains as the right innominate vein, while the communicating branch 

 forms the left innominate. The remnant of the left duct of Cuvier 

 generally remains as a fibrous band, running obliquely down to the coro- 

 nary vein, which is really the proximal part of the left duct of Cuvier. 

 In front of the root of the left lung, another relic may be found in the 



dc 



Fig. 499. Diagrams illustrating the development of the great veins, d c, ducts of Cuvier ;./, 

 jugular veins ; h, hepatic veins ; c, cardinal veins ; , subclavian vein ; j t, internal jugular vein ; 

 je, external jugular vein; a z, azygos vein; c i, inferior venacava; ?, renal veins; il, iliac veins; 

 h ij, hypogastric veins. (Gegenbaur. ) 



form of the so-called vestigial fold of Marshall, which is a fold of peri- 

 cardium running in the same direction. 



In many of the lower mammals, such as the rat, the left ductus 

 Cuvieri 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. 



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

 left superior intercostal and vena azygos minor. 



CIRCULATION OF BLOOD IN THE FOETUS. 



The circulation of blood in the foetus differs considerably from that 

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



