284 HAND-BOOK OF PHYSIOLOGY. 



and revehentes), both opening into the united omphalo-mesenteric and 

 umbilical veins, in such a way that a portion of the venous blood travers- 

 ing the latter is diverted into the developing liver, and, having passed 

 through its capillaries, returns to the umbilical vein through the venae 

 hepaticae revehentes at a point nearer the heart (see Fig. 442). The por- 

 tion of vein between the afferent and efferent veins of the liver becomes 

 the ductus venosus. The venae hepaticae 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. 



FIG. 442. Diagrams illustrating the development of veins about the liver. B, d c, ducts \ f 

 Cuvier, right and left; c a, right and left cardinal veins; o, left omphalo-mesenteric vein; o', rigijt 

 omphalo-mesenteric vein, almost shriveled up; u, u', umbilical veins, of which u', the right one, has 

 almost disappeared. Between the venae cardinales is seen the outline of the rudimentary liver, with 

 its venae hepaticae advehentes, and revehentes; D, ductus venosus; Z', hepatic veins; c i, vena cava 

 inferior; P, portal vein; P' P', venae advehentes; m, mesenteric veins. (Kolliker.) 



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

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

 culation 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 a jugular vein, collecting blood from the head and 

 neck, and a 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 fol- 

 lowing transformations occur. 



As the kidneys are developing a new vein appears (vena cava inferior), 

 formed by the junction of their efferent veins. It receives branches from 

 the legs (iliac) and increases rapidly in size as they grow: further up it 

 receives the hepatic veins. The heart gradually descends into the thorax, 



