382 COMPARATIVE ANATOMY 



the ductus Cuvieri. The blood from the head is returned by the pre- 

 cardinals or jugulars and that from the trunk by the postcardinals. The 

 subclavian, renal, and external iliac branches of the cardinals soon make 

 their appearance. At a later stage, transverse anastomosing vessels 

 connect right and left pre- and postcardinal veins. Another transverse 

 anastomosis is formed in the region of the iliac veins. 



The rapid growth of the liver effects radical changes in the vitelline 

 and umbilical veins which pass through it, both breaking up into capillary 

 networks within the organ. Three transverse connexions between the 

 parallel vitelHne veins are formed. Of these the first and last are dorsal 

 to the duodenum while the middle anastomosis is ventral to the intestine. 

 The united veins thus form a figure 8; but the left half of the anterior loop 

 and the right half of the posterior loop atrophy, and the resultant portal 

 vein assumes the shape of a letter S. With the atrophy of the yolk-sac 

 and its associated vitelline veins, and with the appearance of splenic 

 (Henal) and superior mesenteric branches from the intestine, the portal 

 vein attains its adult relations. The right umbilical vein disappears, 

 while the left umbilical acquires direct connexion with the hepatic portal 

 vein. In this way, a ductus venosus is formed carrying blood directly 

 from the intestine and placenta to the postcava and the heart. (Fig. 319) 



The precardinal veins, which drain the blood from the head, are 

 primarily paired and symmetrical. In an eight week embryo, a transverse 

 anastomosis is established and later converted into the left innominate 

 vein. Eventually the left common cardinal atrophies, but occasionally 

 persists as the oblique vein of Marshall. The left common cardinal 

 persists also as part of the coronary sinus. 



In the older texts it is stated that the posterior cardinals persist as the 

 azygos and hemiazygos veins. The more recent researches of Huntington 

 and McClure, however, indicate that the ontogenesis of the veins below 

 the heart is extremely compHcated. The changes may be more clearly 

 expressed in a series of diagrams of successive stages than from a descrip- 

 tion (see Fig. 320). Postcardinal, subcardinal, and supracardinal 

 veins make their appearances successively in the embryo and acquire 

 complex connexions with one another. The adult postcava is formed by 

 the coalescence and anastomosis of four primarily separate veins, the 

 hepatic, subcardinal, postcardinal, and supracardinal. In addition to 

 these, a sub-supracardinal anastomosis is built into the wall of the post- 

 cava. These changes are seen in the stages represented in Fig. 320. 



Changes in Circulation at Birth. In the fetus oxygenated blood is 

 returned to the heart by the umbilical vein, ductus venosus and the post- 

 cava. In the postcava, venous blood from the lower part of the body 

 mingles with the pure blood from the placenta. Further dilution of the 

 oxygenated blood occurs in the heart by the addition of venous blood from 



