THE DEVELOPMENT OF THE VASCXJLAB SYSTEM. 93 



senting the remains o£ the primitive connection of these 

 arches with the fourth pair and the subvertebral aorta. 

 The dorsal ends of the first, second, and third arches become 

 obliterated ; but their cardiac ends, and the branches which 

 they give off, become the arteries of the head and upper 

 extremities. 



fg' The embryonic aorta gives off omphalomeseraic branches 

 (Fig. 26, o) to the umbilical vesicle; and ends, at first, in the 

 hypofjastric arteries (which are distributed to the allantois 

 in the abranchiate Vertehrata), and a median caudal con- 

 tinuation. The blood from the umbilical vesicle is brought 

 back, as before mentioned, by the omphalomeseraic veins 

 (Pig. 26, o'), which unite in a dilatation close to the head ; the 

 dilatation {sinus venosus) receives, on each side, a short 

 transverse venous trunk, the ductus Cuvieri (Fig. 26, DC). 

 which is itseK formed, upon each side, by the junction of 

 the anterior and posterim- cardinal veins, which run back- 

 wards and forwards, parallel with the spine, and bring back 

 the blood of the head and of the trunk. 



The blood of the allantois is returned by the innbilical 

 vein, or veins (Fig. 26, u'), which are formed in the anterior 

 waU of the abdomen, and open into the venous sinus before 

 mentioned. The blood of the posterior extremities and 

 kidneys is, after a while, brought to the same point by a 

 special median vein, the vena cava inferior (Fig. 26, cv.). 



The development of the liver effects the first great change 

 in the arrangements now described. It, as it were, in- 

 terrupts the course of the omphalomeseraic vein, which is 

 not only the vein of the umbilical sac but also that of the 

 intestine, and converts it into a meshwork of canals, which 

 communicate, on one side, with the cardiac part of the vein, 

 and, on the other side, with its intestinal part. The latter 

 is thus converted into the wma \porfcs (Fig. 26, tip.), dis- 

 tributing the blood of the stomach and intestines to the 

 liver ; while the former becomes the hepatic vein (vh), cariying 

 the hepatic blood to the inferior cava, and thence to the 

 heart. 



