DEVELOPMENT OF THE BLOOD-VASCULAR SYSTEM 



67 



portal vein; while the vessels draining the plexus into the sinus venosus are termed the venae 

 revehentes, and form the future hepatic veins (Figs. 545 and 546). Ultimately the left vena 

 revehens no longer communicates directly with the sinus venosus, but opens into the right vena 

 revehens. The lower part of the portal vein is formed from the fused vitelline veins which 

 receive the veins from the alimentary canal; its upper part is derived from the venous rings by the 

 persistence of the left half of the lower and the right half of the upper ring, so that the vessel 

 .forms a spiral turn round the duodenum (Fig. 546). 



The tivo umbilical veins fuse early to form a single trunk in the body stalk, but remain separate 

 within the embryo and pass forward to the sinus venosus in the side walls of the body. Like the 

 vitelline veins, their direct connection with the sinus venosus becomes interrupted by the inva- 

 sion of the liver, and thus at this stage the whole of the blood from the yolk sac and placenta 

 passes through the substance of the liver before it reaches the heart. The right umbilical vein 

 shrivels and disappears; the left, on the other hand, becomes enlarged and opens into the upper 

 venous ring of the vitelline veins. Finally, a direct channel is established between this ring and 

 the right hepatic vein; this channel is called the ductus venosus, and, enlarging rapidly, it 

 forms a wide channel through whk-h most of the blood, returned from the placenta, is carried 

 directly to the heart without being obliged to pass through the liver. The left umbilical vein 

 and ductus venosus become impervious after birth, and form, respectively, the ligamentum teres 

 and ligamentum venosum of the liver (Fig. 1052). 



Ventral detached portions 

 of umbilical vein.*. 



Venae revehentes. 



Stomach. j 



Venae advehentes. ~ 

 Pancreas. 

 Bile duct. .. 



Obliterated portions 

 of venous rings. 



Right umbilical vein. 



--Ductus venosus. 



umbilical vein. 



Duodenum.' 



Portal vein. 



FIG. 546. The liver, and the veins in connection with it, of a human embryo, twenty-four or twenty-five 

 days old, as seen from the visceral surface. (After His.) (Copied from Milnes Marshall's Embryology.) 



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 sinus 

 venosus. Each of these ducts receives an ascending and descending vein. The ascending 

 veins return the blood from the parietes of the trunk and from the mesonephroi, and are called 

 postcardinal veins. The descending veins return the blood from the head and upper limbs, 

 and are called the precardinal or primitive jugular veins (Fig. 547). The blood from the 

 lower limbs is collected by the right and left iliac veins, which, in the earlier stages of develop- 

 ment, open into the corresponding right and left postcardinals (Fig. 548); later on, a transverse 

 channel (the left common iliac vein) is developed between the caudal parts of the two post- 

 cardinal veins (Fig. 549), and through this the blood is carried to the right postcardinal vein. 

 The portion of the left postcardinal vein below the level of the left renal vein atrophies and dis- 

 appears up to the point of entrance of the left spermatic vein; above this level the left post- 

 cardinal persists as the superior and inferior azygos minor veins. The right postcardinal vein, 

 which now receives the blood from both lower limbs, forms a large venous trunk along the posterior 

 abdominal wall. Above the level of the renal veins the right postcardinal vein persists as the 

 vena azygos major, and receives the right intercostal veins, while the azygos minor veins are 

 brought into communication with it by the development of transverse anastomotic channels in 

 front of the vertebral column (Fig. 515). 



