226 



VASCULAE SYSTEM 



The umbilical veins are for a time double within the abdomen, although 

 they have fused within the umbilical cord into a single trunk. Diverging from this, 

 they pass to the sinus on either side in the somatopleure, just where this is becoming 

 bent round into the amnion (fig. 81, p. 56). After a time, however, it is found that 

 this direct communication with the sinus is partially interrupted by the development 

 of a vascular network, and that on the left side a fresh communication has become 

 established with the upper venous circle of the vitelline veins. The interruption 

 subsequently becomes complete on both sides (fig. 280), and on the right side the 

 greater part of the vein becomes atrophied (on both sides the part which originally 

 opened into the sinus venosus remains evident for a time) (fig. 279). The left vein, 

 on the other hand, increases in bulk with the development of the placental circulation. 

 For a short time the whole of its blood, as well as that of the vitelline vein, passes 

 through the capillaries of the liver. But a branch is soon seen passing from the 

 upper venous circle direct into the right hepatic vein, near its entrance into the 

 sinus. This forms the ductus venosus (Arantii) or vena ascendens, and it now carries 

 most of the blood of the umbilical vein direct to the heart. Subsequently the left 



hepatic vein loses its direct communica- 

 tion with the sinus venosus, and comes 

 to open into the right hepatic where it is 

 joined by the ductus venosus. The 

 channel conveying the blood from the 

 three vessels is called the common hepatic 

 vein, and this vein becomes later con- 

 nected with the vessel which gives rise 

 to the inferior vena cava (see below). 



The lower part of the portal vein is 

 formed, as we have seen, by the united 

 vitelline veins. The upper part is formed 

 as a single trunk out of the double venous 

 annulus by atrophy of the right half of 

 the lower ring and the left half of the 

 upper (fig. 280). The spiral turn around 

 the duodenum is thereby produced, and 

 thus it is also that the portal vein at 

 first appears more directly connected with 

 the right venae advehentes than with the 

 left. Most of these embryonic veins are 

 at first of relatively large size and have an irregular sinus-like character (fig. 279), 

 which disappears at a later stage of development. 



Cardinal veins. On the thirteenth day, two short transverse venous trunks, 

 the ducts of Cuvier, open, as has been above stated, one on each side, into the sinus 

 venosus of the heart. Each is formed by the union of a superior and an inferior 

 vein, named respectively the anterior and the posterior cardinal. 



The posterior cardinal veins are the primitive vessels which return the blood 

 from the Wolffian bodies and body-walls. They receive segmental veins all along 

 their course, and in the region of the Wolffian body are largely broken up into 

 sinus-like spaces (sinusoids). Behind they are continued into vessels which after- 

 wards become the internal iliacs, and these receive, when the limbs develop, the 

 sciatic and then the external iliac veins. 



At first all the blood from the trunk is returned through the cardinals and 

 ducts of Cuvier, but by a series of changes the cardinal system of veins is tapped 

 by the hepatic system, and a new channel is opened up, which becomes the inferior 

 vena cava. In the region of the Wolffian bodv the cardinals receive a series of 



FIG. 281. UNDER-SURFACE OF THE FCETAL 



LIVER, WITH ITS GREAT BLOOD-VESSELS, AT 

 THE FULL PERIOD. 



a, the umbilical vein, lying in the umbilical 

 fissure, and turning to the right side, at the 

 transverse fissure (o), to join the vena portse (p) ; 

 d, the ductus venosus, continuing straight on to 

 join the vena cava inferior (c) ; some branches 

 of the umbilical vein pass from a into the 

 substance of the liver ; g, the gall-bladder, cut. 



