348 THE CAT. [CHAP. x. 



ment of the pla cental circulation, the only important veins besides 

 the cardinal ones are the omphalo-meseraic veins ; but with the 

 beginning of the placental circulation two veins, called the right and 

 left umbilical veins (iiv), come from the placenta, pass up the allantois 

 and join the omphalo-meseraic veins at their entrance into the 

 liver. The right omphalo-meseraic vein and the right umbilical 

 vein soon disappear altogether. The left omphalo-meseraic vein 

 sinks by degrees into comparative insignificance, but is joined by 

 tbe now more important mesenteric veins. Of the vessel thus formed, 

 that part which is on the heart side of the liver, ultimately becomes 

 the vessel connecting the hepatic veins with tbe heart. The left 

 (now the only) umbilical vein, joined by veins from the anterior 

 abdominal walls and other parts, unites with what was the common 

 omphalo-meseraic trunk, now become the portal vein, and also 

 sends out branches (portal veins) into the growing liver, while much 

 of the blood it brings is conveyed on by an undivided vein, called 

 the ductus venosus (Fig. 156, dv), directly to the sinus venosus. 

 Meantime other veins the hepatic veins (hv) form in the substance 

 of the liver and open close beside the opening of the ductus venosus 

 into the sinus venosus. Thus what was originally the combined 

 trunks of the omphalo-meseraic and umbilical veins persists till birth 

 as the ductus venosus, while diversely directed branching veins 

 (forming two systems, the portal and the hepatic), are developed in 

 connection with it the portal system of veins diverging into the 

 liver from near the proximal end of the ductus venosus, while the 

 hepatic veins converge towards the vicinity of the distal end of the 

 ductus venosus. 



Meanwhile, concomitantly with the incipient development of the 

 pelvic limbs, there arises the great system of the vena cam inferior 

 (vci). This is formed by the junction of the veins of the legs and 

 pelvic viscera (the incipient external and internal iliac veins) into a 

 median trunk, which receives accessions from the kidneys (renal 

 veins), and runs on beneath the great dorsal aorta to the heart, 

 where, intruding upon the junction of the hepatic veins and sinus 

 .venosus, it ultimately absorbs, as it were, the latter chamber alto- 

 gether, and thus the hepatic veins come in the adult to open into the 

 vena cava inferior. 



During this period, the right ductus Cuvieri becomes attached to 

 the developing right auricular part of the heart, and while the 

 anterior cardinal veins become large and important, as the jugular 

 and subclavian veins, the posterior cardinal veins become relatively 

 reduced into the right and left vena azygos. Then a transverse 

 communicating branch, which connects the two jugular veins, 

 rapidly enlarges, while the left ductus Cuvieri diminishes and dis- 

 appears. This transverse branch becomes the left innominate 

 vein, and thus the right ductus Cuvieri (having now the right 

 jugular and subclavian veins brought into connexion with it) be- 

 comes transformed into the vena cava superior. The right azygos 

 vein (the old right posterior cardinal vein) opens at last, as at first, 



