DEVELOPMENT. 693 



liver, and, having passed through its capillaries, returns to the umbili- 

 cal vein through the venae hepaticse revehentes at a point nearer the 

 heart (see Fig. 479). The portion of vein between the afferent and 

 efferent veins of the liver becomes the ductus venosus. The venae he- 

 paticae advehentes become the right and left branches of the portal vein, 

 the venae hepaticae revehentes become the hepatic veins, which open just 

 at the junction of the ductus venosus with another large vein (vena cava 

 inferior), which is now being developed. The mesenteric portion of the 

 omphalo-mesenteric vein returning blood from the developing intestines 

 remains as the mesenteric vein, which, by its union with the splenic 

 vein, forms the portal. 



Thus the foatal liver is supplied with venous blood from two sources, 

 through the umbilical and portal vein respectively. At birth the circu- 

 lation through the umbilical vein of course completely ceases and the 

 vessel begins at once to dwindle, so that now the only venous supply of 

 the liver is through the portal vein. The earliest appearance of the 

 parietal system of veins is the formation of two short transverse veins 

 (ducts of Cuvier) opening into the auricle on either side, which result 

 from the union of an anterior cardinal, afterwards forming a jugular, 

 vein, collecting blood from the head and neck, and a posterior cardinal 

 vein which returns the blood from the Wolffian bodies, the vertebral 

 column, and the parietes of the trunk. This arrangement persists 

 throughout life in Fishes, but in Mammals the following transforma- 

 tions occur. 



As the kidneys are developing a new vein appears (vena cava infe- 

 rior), formed by the junction of their efferent veins. It receives 

 branches from the leg (iliac) and increases rapidly in size as they grow: 

 further up it receives the hepatic veins, which by now have lost their 

 original opening into the ductus venosus. The heart gradually descends 

 into the thorax, causing the ducts of Cuvier to become oblique instead 

 of transverse. As the fore-limbs develop, the subclavian veins are 

 formed. 



A transverse communicating trunk now unites the two ducts of 

 Cuvier, and gradually increases, while the left duct of Cuvier becomes 

 almost entirely obliterated (all its blood passing by the communicating 

 trunk to the right side) (Fig. 480, c. D.). The right duct of Cuvier re- 

 mains as the right innominate vein, while the communicating branch 

 forms the left innominate. The remnant of the left duct of Cuvier gen- 

 erally remains as a fibrous band, running obliquely down to the coronary 

 vein, which is really the proximal part of the left duct of Cuvier. In 

 front of the root of the left lung, another relic maybe found in the form 

 of the so-called vestigial fold of Marshall, which is a fold of pericardium 

 running in the same direction. 



