THE MODIFICATIONS OF THE VASCULAR SYSTEM. 95 



arteries have become intestinal arteries, and tlae omphalo- 

 meseraic vein, the vena porta3. The hypogastric arteries are 

 obliterated, except so much of them as is converted into the 

 common iliac arteries. The umbilical vein, or veins, also 

 disappear, or are represented by mere ligaments. 



Of the three veins which open into the venous sac — viz., the 

 inferior cava, and the right and left chictus Cuvieri, — all may 

 persist, the latter receiving the title of right and left superior 

 cavcB. Or, as very often happens in the higher Vertehrata, 

 the left ductus Cuvieri becomes more or less obliterated ; the 

 veins which properly open into it acquiring a connection 

 with the riffht ductals, which then remains as the sole 

 superior cava. The posterior cardinal veins give oif anas- 

 tomosing branches, which are converted into the vencB 

 azygos ; the anterior cardinal veins become metamoi-phosed 

 into the external jugular veins and vence innominatce. 



In Fishes, the sinus venosus and the cardinal veins persist 

 throughout life ; but the anterior cardinal veins, which 

 bring back the blood from the head and from the anterior 

 extremities, are called veruB jugulares. 



The ca.udal veins are either directly continued into the 

 cardinal veins, as in Marsipohranchii and Elasmobranchii, or 

 branch out into the kidneys, as in many Teleostei. In either 

 case the efferent renal veins open into the cardinal veins. 



The portal veins, conveying the blood of the chylopoietic 

 viscera, and sometimes that of other organs and of the abdo- 

 minal walls, maybe one or many. In Amphioxus a,nd Myxine 

 the vein is rhythmically contractile, and forms a portal 

 heart. 



In most Amphibia and Beptilia the simis venosus persists, 

 and is rhythmically contractile, valves being placed at its 

 opening into the right auricle. 



The anterior cai'dinal veins are represented by jugular 

 veins, the posterior cardinal by vertebral veins ; these, and 

 the veins of the anterior extremities, when they are 

 present, pour their blood into the ductus Cuvieri, which are 

 now termed anterior vence. cavcB. 



The vena cava inferior takes its origin chiefly by the 



