8 3 6 



DEVELOPMENT. 



[CH. LVIII. 



they are for a long time considerably larger than the external 

 iliacs which supply the comparatively small hind-limbs. 



Veins. The earliest veins to appear in the foetus are the omphalo- 

 mesenteric or vitelline, which return the blood from the yolk-sac to 

 the developing auricle. As soon as the placenta with its umbilical 

 veins is developed, these unite with the omphalo-mesenteric, and 

 thus the blood which reaches the auricle comes partly from the 

 yolk-sac and partly from the placenta. The right omphalo- 

 mesenteric and the right umbilical veins soon disappear, and the 

 united left omphalo-mesenteric and umbilical veins pass through 

 the developing liver on the way to the auricle. Two sets of 

 vessels make their appearance in connection with the liver (venae 

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



o',- 



Fig. 656. Diagrams illustrating the development of veins about the liver. B, d, c, ducts 

 of Cuvier, right and left; c a, right and left cardinal veins ; o, left omphalo-mesenteric 

 vein ; o', right omphalo-mesenteric vein, shrivelled lip ; u it', umbilical veins, of 

 which u', the right one, has disappeared. Between the vense cardinales is seen the 

 outline of the rudimentary liver with its vense hepaticoe advehentes and revehentes. 

 D, later stage ; I, ductus venosus ; V, hepatic veins ; c i, vena cava inferior ; P, portal 

 vein ; P' F 1 , v^nec advehentes ; TO, mesenteric veins. (Kolliker.) 



mesenteric and umbilical veins, in such a Avay that a portion of the 

 venous blood traversing the latter is diverted into the developing 

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

 umbilical vein through the vense revehentes at a point nearer the 

 heart (see fig. 656). The portion of vein between the afferent 

 and efferent veins of the liver is called the diictus venosus. The 

 vense advehentes become the right and left branches of the portal 

 vein, the vense 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 foetal liver is supplied with venous blood from two 



