838 



DEVELOPMENT. 



[CH. LVIII. 



A transverse communicating trunk now unites the two primi- 

 tive jugular veins, 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. 657, c. D.). 

 The right primitive jugular vein remains as the right innominate 

 vein, while the communicating branch forms the left innominate. 

 The right duct of Cuvier becomes the superior vena cava. The 

 remnant of the left duct of Cuvier generally remains as a fibrous 



Renal 



Hepatic Veins 



Left Common Iliac 



Ext. Iliac 



Sciatic 



Fig. 658. 



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 peri- 

 cardium running in the same direction. 



In some of the lower mammals, such as the rat, the left duct of 

 Cuvier 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. 



