ORGANS OF CIRCULATION. 1 97 



The blood distributed by the hypogastric arteries is returned 

 to the heart by the derivatives of a pair of hypogastric veins 

 which run on the ventral body wall forward to the omphalomes- 

 araic vein. When the hind limbs appear, external and internal 

 iliac veins grow out from the hypogastrics into those appendages, 

 their ultimate distribution coinciding more or less closely with 

 the similarly named arteries. When the posterior cardinals grow 

 back into this region they tap these vessels, and so the blood 

 from the hinder appendages is returned to the heart through 

 them, at first directly, later through the renal portal system 

 (Fig. 206), and in the higher vertebrates by way of the post- 

 cava. The ventral portions of the hypogastric veins retain their 

 connection with the iliacs throughout life in the ichthyopsida 

 (Fig. 206), and either as two vessels or as a single anterior 

 abdominal vein, run forward in the ventral body wall, and enter 

 the portal system (Fig. 203). 



In the amniotes, with the formation of the allantois, the hy- 

 pogastric veins grow out into this, and are here known as the 

 umbilical veins. In the reptiles they retain their distinctness ; 

 but in birds and mammals one aborts, leaving the other as a 

 single trunk which empties into the omphalomesaraic. Dur- 

 ing embryonic life this system is very large and important, but 

 after hatching or birth it becomes reduced to an inconspicuous 

 condition, Fig. 203. 



In the fishes the relations of jugulars and the ducts of 

 Cuvier are much as outlined above, with the exception that the 

 jugular veins develop two branches, internal and external. With 

 the formation of lungs (dipnoi and amphibia) this system be- 

 comes unsymmetrical, in that the left Cuverian duct is now 

 compelled to reach the right side of the sinus venosus ; and here, 

 as in the higher groups, the trunks, formed of united jugulars, 

 subclavians, and posterior cardinals (i.e., the Cuverian ducts), 

 are known as the precavae, right and left. Here, too, is to be 

 noticed a shifting of the veins (subclavians) coming from the 

 fore limbs. At first they empty into the posterior cardinals, but 

 later they empty into the jugulars, the common trunks formed 

 by the subclavians and jugulars being known as the innominate 

 veins. In the birds a transverse anastomosis forms between the 



