THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



263 



has already been noted, the omphalomesenteric veins enter the body at the 

 umbilicus, pass cranially along the intestine and open into the caudal end of the 

 heart. The umbilical veins, which appear soon after, enter the body at the 

 umbilicus and pass cranially, one on each side, in the ventro-lateral part of the 

 body wall; at the level of the heart they turn mesially through the septum trans- 

 versum and join the corresponding omphalomesenteric veins to form a common 

 trunk on each side, into which the duct of Cuvier then opens (Fig. 231). 

 When the liver grows out as an evagination from the intestine, it comes in con- 

 tact with the proximal ends of the omphalomesenteric veins and, as it enlarges, 

 breaks them up into numerous smaller channels (Fig. 237). 



(Esophagus 



Ant. cardinal 



Duct of Cuvier 

 Left umbilical 

 Duct us venosus 



Left umbilical 



Venous ring 

 Venous rin 



Omphalomesenteric 

 Intestine 



FIG. 238. Veins in the liver region of a human embryo of 4 mm. His, Kollmann's Atlas. 



The blood then, instead of having a direct channel, is forced to flow through 

 these smaller channels which have been termed sinusoids. When the liver has 

 attained a considerable size a more direct and definite channel is formed, 

 which extends through the substance of the liver from the proximal end of the 

 right omphalomesenteric vein obliquely caudally to the left omphalomesenteric 

 vein. This newly formed channel is the ductus venosus (Figs. 237 and 238). 

 In the meantime, three transverse anastomoses develop between the omphalo- 

 mesenteric veins just caudal to the liver. The middle one is dorsal to the 

 intestine, the other two ventral, so that the intestine is surrounded by two venous 

 loops or rings (Figs. 237 and 238). At the same time a cross-anastomosis 

 develops between the left umbilical vein, which is primarily the smaller, and 



