GENERATION AND DEVELOPMENT. 



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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. 443, 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 re- 



sv 



FIG. 443. Diagrams illustrating the development of the great veins, d c, ducts of Cuvier; j, 

 jugular veins ; /i, hepatic veins; c, cardinal veins; s, subclavian vein; j i, internal jugular vein; j e, 

 external jugular vein; a z, azygos vein; ct, inferior vena cava; r, renal veins; i I, iliac veins; h ij, 

 hypogastric veins. (Gegenbaur.) 



mains as a fibrous band, running obliquely down to the coronary 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 form of the 

 so-called vestigial fold of Marshall, which is a fold of pericardium 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 cardinal vein into the right; and by this union the great azygos vein 

 is formed. 



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

 intercostal and vena azygos minor (Fig. 443, D). 



