152 



DEVELOPMENT OF THE PKINCIPAL VEINS. 



VTu.5, 



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

 blished with the upper venous circle of the ritelline veins. The interruption subse- 

 quently becomes complete on both sides (fig. 183), and on the right side the greater 



Fig. 183. VENOUS TRUKKS OF A HUMAN EMBRYO OP 



ABOUT THREE-AND-A-HALF WEEKS. (His.) 



v.c.d, v.c.s, superior venae cavse, right and left; 

 v.j, v.c, primitive jugular and cardinal vein ; 

 v.u.d, v.u.s, umbilical veins, right and left ; v.u", 

 v.u", upper detached portions of umbilical veins ; 

 v.v.om, omphalomeseraic or vitelline veins forming 

 the vena portse. The permanent veins are coloured 

 blue. 



part of the vein becomes atrophied (on 

 both sides the part which originally opened 

 into the sinus reuniens remains evident 

 for a time). The left vein, on the other 

 hand, increases in bulk with the develop- 

 ment 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 or vena 

 ascendens, and it now carries most of the blood of the umbilical vein direct to the 

 heart. Subsequently the direct communication of the left hepatic vein with the 



Fig. 184. UNDER SURFACE OF THE F<ETAL 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 trans- 

 verse fissure (o), to join the vena portae ( p) ; rf, the 

 ductus venosus, continuing straight on to join the 

 vena cava inferior (c) ; some branches of the umbili- 

 cal vein pass from a into the substance of the liver ; 

 y, the gall-bladder, cut. 



sinus becomes obliterated, and a new com- 

 munication becomes established with the 

 ductus venosus ; and, finally, when, with 

 the growth of the lower limbs and of the 



other abdominal and pelvic organs, the inferior vena cava becomes developed, this 

 also joins the upper end of the ductus venosus. 



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. 183). The spiral turn around the duodenum is thus produced, and thus it is 

 also that the portal vein at first appears more directly connected with the right vense 

 advehentes than with the left. 



Most of these embryonic veins are at first of relatively large size and have an 

 irregular sinus-like character, which disappears at a later stage of development. 



At the time of commencement of the placental circulation, two short transverse 

 venous trunks, the ducts of Cuvier, open, as has been above stated, one on each side, 

 into the auricle of the heart. Each is formed by the union of a superior and an 

 inferior vein, named respectively the primitive jugular and the cardinal. 



