THE EMBRYO. 



1195 



shrivels up and almost entirely disappears ; the left, on the other hand, becomes 

 much enlarged after the establishment of the placental circulation, and opens into 

 the upper venous ring. Finally a direct branch is established between this ring 

 and the right hepatic vein ; this branch is the ductus venosus or vena ascendent, 

 and, enlarging rapidly, it forms a wide channel through which most of the blood, 

 returned from the placenta, is carried direct to the heart (Fig. 754). 



The Parietal Veins. The first indication of a parietal system consists in the 

 appearance of two short transverse veins (the ducts of Cuvier), which open, one 

 on either side, into the auricular portion of the heart. Each of these ducts is 

 formed by an ascending and a descending vein. The ascending veins return the 

 blood from the parietes of the trunk and from the Wolffian bodies, and are 

 called cardinal veins. The descending veins return the blood from the head, 



Right primitive jugular vein. 

 Right cardinal vein.-. 



Bight duct of Cuvier. - 



<Siims venosus. 

 Right hepatic vein.- 



Portal vein, .^t 



Portal vein.- 

 . Right umbilical vein.. 



I 



Umbilical corrf.-/__ 



Left primitive 

 ,'' jugular vein. 



--Left cardinal vein. 



'Left duct of Cuvier. 



-Left hepatic vein. 

 fi-Left umbilical veins. 



Left umbilical vein. 



FIG 753.-Human embryo with heart and anterior body wall removed to show the sinus venosus and its 

 tributaries. (From Kollmanu's Entwickelungsgeschichte.) (After His.) 



arid are called primitive jugular veins (Fig. 750). The blood from the lower 

 limbs is collected by the iliac veins, which empty themselves into the cardinal 

 veins. In the earlier stages of development the right and left iliac veins 

 open into ihe corresponding right and left cardinal veins (Fig. 756), but later < 

 a transverse branch connects the lower ends of the two cardinal veins, and through 

 this the blood from the left iliac vein is carried into the right cardinal vein. 

 By the development of a similar transverse branch higher up the blood from t 

 left kidnev is also carried into the right cardinal vein (Fig. 755, 2). The portion 

 of the feft cardinal vein above the origin of the lower transverse branch 

 becomes atrophied as high as the level of the renal vein, above which it persists 

 as the vena azygos minor. The right cardinal vein, which now receives 

 blood from both lower extremities, forms a large venous trunk along the posterior 



