704 MANUAL OF PHYSIOLOGY. 



advehens), and forms the main trunk of the portal vein (Fig. 

 290, B. and D.). 



At this stage of the formation of the veins there are three 

 trunks opening into the auricle the right and left ducts of 

 Cuvier and the inferior vena cava. 



As development proceeds, the lower parts of the cardinal veins 

 join the external iliac veins, forming the common iliacs, and so 

 return their blood into the inferior vena cava. 



The upper parts of the cardinal veins become continuous with 

 the posterior vertebral veins which convey the blood from the 

 parietes of the embryo. Between the latter a communicating 

 branch is established, which helps in the formation of the azygos 

 vein. 



The ducts of Cuvier, which at first were placed almost at right 

 angles to the auricle, become more oblique in their direction as 

 the heart descends. 



Between the primitive jugular veins a cross branch is developed, 

 which conveys the blood from the left side of the head and upper 

 extremity to the duct of Cuvier of the opposite side. 



The left duct of Cuvier, below the communicating branch, 

 atrophies and forms part of the coronary veins of the heart ; 

 the connection between this and the vein above the cross branch 

 being, in the adult, represented by a small vein, or a band of 

 fibrous tissue, called the vestigial fold of the pericardium. 



The cross branch from the left to the right jugular becomes 

 the left innominate vein. The right duct of Cuvier and the right 

 jugular below the entrance of this cross branch forms the supe- 

 rior vena cava, whilst the part of the right primitive jugular 

 immediately above the entry of the left innominate vein forms 

 the right innominate vein. 



The posterior vertebral vein of the right side forms the vena 

 azygos major ; the corresponding branch of the opposite side, 

 together with the part of the left primitive jugular below the 

 cross branch, forms the left superior intercostal vein and the supe- 

 rior vena azygos minor. The lower part of the left posterior 

 vertebral vein, together with the connecting branch to the right 

 vein, remain as the inferior vena azygos minor. 



