282 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



mesonephroi, posterior extremities and dorsal body wall is in early stages drained 

 by the posterior cardinal veins alone. 



The development of the unpaired vena cava inferior begins when communica- 

 tion is established between the right hepatic vein of the liver and the right sub- 

 cardinal vein of the mesonephros, primarily a tributary of the posterior cardinal 

 vein (Lewis, 1902). 



The liver on the right side becomes attached to the dorsal body wall and from 

 its point of union a ridge, the plica vena? cavae (Fig. 192), extends caudalward. 

 According to Davis (1910), capillaries from the subcardinal vein invade the plica 



Confluence off he sinuses 



icJdle cerebral ve.m 



Tosterior cerebral 

 i/etn 



Superior petrosal 

 sinus 

 Transverse, Sinus 



Ophthalmic vem 



Ophthalmic vein 



jolirory vesicle 

 Inf. petrosal sinus 

 Trigeminal nerve 



Fig. 276. — Four diagrams showing the development of the veins of the head (after Mall). A, at four 

 weeks; B, at five weeks; C, at the beginning of the third month; D, from an older fetus. 



venae cavae and, growing cranially, meet and fuse with capillaries extending 

 caudad from the liver sinusoids. 



Thus is formed the vein of the plica vence cavce, which is already present in 

 human embryos of 2.6 mm. (Kollmann). This vein rapidly enlarges as also do 

 the sinusoidal connections between the subcardinals and posterior cardinals at 

 one point. Thus the blood from the lower posterior cardinals is soon carried to 

 the heart, chiefly by way of the right subcardinal and right hepatic veins (Fig. 

 B, 274 ). Soon the posterior cardinals just cranial to their enlarged anastomoses 

 with the subcardinals become small and are interrupted. Cranial to their inter- 



