71 



Basic Struct lire of Vertebrates 



Fig. 74. Relations and modifications of the post- and subcardinal, abdominal, 

 and postcaval veins in successive developmental stages of an amphibian. (A) The 

 veins (il) from the hindlimb return directly to the heart by the lateral abdominal 

 veins (la), while the blood from the tail (c) passes by way of the subcardinals (sc) 

 through the mesonephroi to the postcardinals (pc). (B) The lateral abdominals 

 have united in front to form the anterior abdominal vein (aa); the iliacs have sent 

 a branch to the postcardinals, which have grown back to join the caudal, while the 

 subcardinals have lost their connection with the caudal and have acquired one 

 with the postcava (p), a backward growth from the sinus venosus. (C) The post- 

 cardinals have been interrupted, the posterior half of each now forming an advehent 

 renal vein (av) while the subcardinals, as in B, form the revehent veins (r). (cd) Duct 

 of Cuvier (precava); (h) hepatic vein; (J) jugular vein. (Courtesy, Kingsley: "Com- 

 parative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



Hepatic-Portal System. A highly unique feature of the arrange- 

 ment of the veins in a vertebrate exists between the digestive tube and 

 the liver. In general, the blood, in the course of its round trip from 

 heart to tissues and back to heart, passes through only one system of 

 capillaries. But the blood which is delivered by arteries to the walls of 

 the coelomic portion of the digestive tube, having passed into capillaries 

 in the tissues of the tube, is drained away by veins which convey it 

 directly into the liver and distribute it into capillaries throughout the 

 entire mass of the liver (Fig. 64). From these hepatic capillaries the 

 blood is re-collected by the hepatic veins, which then take it directly 

 to the heart. The blood, therefore, in its circuit from heart to heart, has 

 passed through two systems of capillaries. This peculiar set of veins 

 interposed between the digestive tube and liver is called the hepatic- 

 portal system. The arrangement enables the liver to remove from the 



