VI.] THE VEINS OF THE LIVER. 171 



indicative of branches about to be formed. These are on the 

 fourth day actually formed, and become connected with the 

 capillary network simultaneously developed in the hepatic 

 substance in such a way that those branches which come off 

 from the ductus venosus soon after its entrance between the 

 liver-lobes, carry blood into the substance of the liver, while 

 those which join the ductus venosus shortly before it leaves the 

 liver, carry blood away from the hepatic substance into the 

 ductus. The former are called vence advehentes, the latter 

 venm revehentes. As a result of this arrangement, there is a 

 choice of paths for the blood in passing from the omphalo- 

 mesaraic vein to the sinus venosus ; it may pass through the 

 capillary network of the liver, going in by the venae adve- 

 hentes, and coming back again by the venae revehentes, or it 

 may go straight through the ductus venosus without passing 

 at all into the substance of the liver. 



As the alimentary canal by its continued closing in 

 becomes on the fourth day more and more distinct from the 

 yolk-sac, it gradually acquires veins of its own, the mesenteric 

 veins, which first appear as small branches of the omphalo- 

 mesaraic vein, though eventually, owing to the change in the 

 relative size and importance of the yolk-sac and intestine, the 

 latter seems to be a branch of one of the former. 



Corresponding to the increase in the size of the head, the 

 superior cardinal veins (Fig. 53, J.) become larger and more 

 important and are joined by the superior vertebral (Su.V.) 

 and wing veins (W). As before, they form the ductus 

 Cuvieri (B.C.) by joining with the cardinal veins (G). 



The latter are now largely developed ; they seem to take 

 origin from the Wolffian bodies, and their size and importance 

 is in direct proportion to the prominence of these bodies. 

 They might be called the veins of the Wolffian bodies. 



As the kidneys begin to be formed, a new single median 

 vein makes its appearance, running from them forwards, 

 beneath the vertebral column, to fall into the sinus venosus 

 (Fig. 53, V.C.I). This is the vena cava inferior. 



As the lungs are being formed, the pulmonary veins also 

 make their appearance and become connected with the left 

 side of the auricular division of the heart. 



The blood carried to the allantois by the umbilical 

 arteries is brought back by two veins w r hich very soon after 



