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 venae cavse (Fig. 192), extends caudalward. 

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



Confluent* oftht sinuses 



Middle cerebral Vein 



Posterior cerebral 

 vein 



Vena. .... 

 cap/r/5 



/! I. -i- laTerans 

 Auditory 



Middle cerebral veut 



ofth 



Anterior cerebral vein 

 Confluen 



,, *!" 

 Superi 

 Jugift 

 Sinus 



Tha*m 



V<rna ca'pttls 

 Ophthalmic vein lateraJis 



'nerve 

 Inferior sayiltal sinus Sfraiykt Sinus 



'uqular 



uqu 

 l/ei 



D 



Middle. 

 cereora.1 



Vein 

 Anterior 

 cerebral 



SphenonarietaJ 

 r (smus 



Confluence 

 of the 

 Sinuses 



uptriOT petroiai 



Sinus 

 Transverse Sinus 



Audi rory vesicle 



Ophthalmic vein \ In f- f*lro,al sintu 

 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 vena caves, 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- 



