THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



259 



subcardinal vein a short distance cranial to the above mentioned anastomosis 

 (Fig. 232, A and B). This branch forms the proximal part of the inferior vena 

 cava. At the same time, also, each subcardinal forms a direct connection with 

 the corresponding cardinal at a point opposite the first anastomosis; consequently 

 the inferior vena cava, the subcardinals and the cardinals are all in direct 

 communication (Fig. 232, B). Thus two ways are formed by which the 

 blood may return to the heart : It may return via the cardinals and ducts of 

 Cuvier, and via the inferior vena cava. 



It is obvious that while these conditions exist, that is, while the mesonephros is functional, 

 and blood is carried to it by the cardinal veins and from it by the subcardinal veins, there 

 is a true renal portal system. The blood from the body walls and lower extremities is col- 

 lected by the segmental vessels and poured into the cardinal veins and is then distributed in 

 the mesonephros by smaller channels or sinusoids (Minot), whence it is collected and carried 

 off by the subcardinal veins. This passage of blood through purely venous channels simu- 

 lates the conditions in the liver where there is a true hepatic portal system. 



Fig. 234.— Diagram of final stage in the development of the superior vena cava 

 and the azygos vein. (Compare with Fig. 233.) 



From this time on, the changes are largely regressions in the cardinal and 

 subcardinal systems, corresponding to the atrophy of the mesonephroi, and 

 rapid increase in the vena cava and its branches. The cranial end of each 

 cardinal becomes smaller; the left loses its connection with both the vena cava 

 and the duct of Cuvier, the right its connection with the vena cava only (Fig. 

 234). Subsequent changes in these parts of the cardinals will be considered 

 in the following paragraph. For a time the caudal ends of the two cardinals 

 are of equal, importance. Later, however, the right becomes larger, while the 

 left atrophies. The right thus becomes a direct continuation and really a part 

 of the vena cava (Figs. 233 and 236). This is brought about, of course, by the 



