VENOUS SYSTEM. 707 



however, beyond the scope of this work to describe in detail the 

 origin of the lesser vessels. 



Venous System. The blood is returned from the head by the 

 two primitive jugulars, which unite with the cardinal veins con- 

 veying the blood from the trunk and lower extremities to form a 

 vessel on each side, called the duct of Cuvier. 



From the lower extremity of the embryo the inferior vena cava 

 commences by the union of the external iliac veins ; this passes up 

 and opens into the venous trunk common to the left vitelline and 

 left umbilical veins. 



The left vitelline becomes continuous with the vessels from the 

 common trunk going to the right side of the liver (the right vena 

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

 290, B. & 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 



