DEVELOPMENT OF THE VEINS. 885 
sides of the duodenum. Three transverse anastomoses soon form between them, of which 
the lower and the upper are in front of and the middle is behind the duodenum ; thus 
two vascular circles are formed round that portion of the gut. Whilst the loops are being 
formed the liver grows rapidly, and it interrupts the direct connexion of the vitelline 
veins with the heart. When the upper vascular loop is established the two vitelline veins, 
which appear to be prolonged from its sides, enter the liver and break up into capillaries, 
from which two new vessels arise which carry the blood on to the sinus venosus. The 
veins which carry the blood to the liver are now known as the vene advehentes ; they 
become the right and left divisions of the 
portal vein. The vessels which carry the  / 
blood to the sinus venosus are the vene  \— \. 
revehentes, and they become the hepatic mt 
veins. h 
From the inferior part of the lower 
venous circle the vitelline veins fuse into 
a common stem which receives the veins 
from the abdominal portion of the gut \ | 
and becomes the commencement of the jo > 
portal vein. The remainder of the portal 
vein is formed by the transverse com- 
munications previously described, together 
with the left half of the lower and the 
right half of the upper circle ; the opposite 
halves of these circles disappear. Thus a 
continuous single stem is formed, which, 
after the disappearance of the yolk-sac, /) 
receives blood from the abdominal part aN \ Vena advehens 
of the alimentary canal, and terminates 
above in the liver. 
The allantoic or umbilical vems com- 
mence in the placenta and fuse into a 
single stem which traverses the umbilical 
cord to reach the embryo, and divides at 
the umbilicus into right and left divisions. ——— 
The two veins then pass through the wa 
septum transversum, one to the right and 
the other to the left, and open into the 
sinus venosus. After a very short period 
the communications with the sinus are 
obliterated, and both veins end in the 
venous capillaries of the liver, the blood 
they convey now passing to the sinus 
venosus by the hepatic veins. This con- i, 
dition also is only transitory, for the right 
vein soon undergoes complete atrophy and 
disappears, whilst the left opens into the 
upper loop of the vitelline veins. In the 
meantime, however, another channel, the 
ductus BCPORUS, has been developed, nels Stage II.—The vitelline and umbilical veins terminate in 
oSsce directly from the pe loop of the he iia and transverse anastomoses have formed 
vitelline veins, or rather from the left between the vitelline, cardinal, and primitive jugular 
vena advehens to the right hepatic vein, veins of opposite sides. 
and by this channel the ereater part of 
the placental blood passes to the heart without traversing the liver substance. Some of 
the placental blood, however, goes to the liver by the left vena advehens. Finally the left 
vena revehens loses its connexion with the sinus venosus and opens into the right vena 
revehens. The left umbilical vein and the ductus venosus remain pervious until birth, 
when the placental circulation ceases. The two vessels then rapidly atrophy, and are 
subsequently represented by fibrous cords, of which that formed from the left umbilical 
vein is known as the round ligament of the liver. 
The primitive jugular and cardinal veins, and the ducts of Cuvier, which constitute the 
main veins of the Wolffian body and of the body wall, head, neck, and limbs of the embryo, 
are represented in the adult by the external jugular veins, the whole of the right and part 
Lateral sinus 
External jugular vein 
Internal jugular vein 
Subclavian vein 
Transverse anastomosis 
Lower part of primitive 
jugular vein 
Duct of Cuvier 
Cardinal vein 
Transverse anastomosis 
Vena revehens 
Umbilical vein 
Stem formed by fused 
vitelline veins 
——— Renal vein 
Transverse anastomosis 
External iliac vein 
Fic. 604.—DEVELOPMENT OF THE VENOUS SYSTEM 
(Diagrammatic). 
