928 



HUMAN ANATOMY. 



DC 



A 



DC 



DC 



those of the renal veins. The portion of the right subcardinal which Hes anterior to the cross- 

 branch then enlarges and unites with the existing portion of the inferior vena cava, and the lower 

 portion of the right cardinal, together with the portion of the cross-branch which intervenes 

 between it and the right subcardinal, also enlarges, and these three elements eventually come 

 into line with one another and with the terminal portion of the ductus venosus to form the 

 inferior vena cava. 



The lower portions of both subcardinals now degenerate, and the upper portion of the left 

 vein, diminishing in size, becomes the left suprarenal vein. A cross-branch forms between 

 the two posterior cardinals at the level of the common iliac veins, and the lower part of the left 

 cardinal then disappears (Fig. 778, C), a small portion below the original renal cross-brancn 

 alone persisting to form the terminal part of the left spermatic (ovarian) vein, which thus comes 

 to open into the left renal vein, since the terminal portion of that vessel represents the original 

 renal cross-branch. As a result of this degeneration the left common iliac vein comes to open 

 into the lower part of the right posterior cardinal by way of the iliac cross-branch. 



The Azygos Veins. — While these changes have been taking place the anterior portions of 

 the posterior cardinals have undergone degeneration immediately anterior to the renal cross- 

 branch (Fig. 778), so that they form independent vessels, receiving the intercostal veins and 

 terminating above in the ductus Cuvieri. They now constitute the azygos veins, and, on the 



degeneration of the lower part of the left 

 Fig. 779. anterior cardinal, the left azygos develops 



connection with the right by one or two 

 transverse branches and then separates 

 from the coronary sinus, the adult condi- 

 tion of the hemiazygos vein being thus 

 acquired. 



The Portal Vein.— Passing along the 

 umbilical cord to the body of the embryo 

 are two vitelline or omphalo-mesenteric 

 veins, which have their origin in the yolk- 

 sac, and the mnbilical vein, which brings 

 back the blood from the placenta. When 

 it reaches the umbilicus, the umtnlical 

 vein divides into two stems which pass 

 upward upon the inner, surface of the 

 anterior abdominal wall and unite above 

 with the corresponding vitelline veins to 

 open into the ductus Cuvieri. The vitel- 

 line veins pass from the umbilicus to the 

 intestine and ascend along it, receiving 

 tributaries from it ; above, they traverse 

 the liver, breaking up into a net-work in its 

 substance, and are then continued on to 

 unite with the umbilicals (Fig. 779, A). 

 By the enlargement of certain portions of 

 the hepatic net-work a well-marked venous 

 stem is formed, extending from the point 

 where the left vitelline vein enters the liver 

 to the junction of the common stem formed 

 by the right vitelline and umbilical veins 

 with the duct of Cuvier. This new stem is 

 the ductus venosus (Figs. 779, ^5" and C), 

 and it later forms a connection with the left umbilical vein and becomes the continuation 

 of that vessel as the result of the degeneration of the upper part of the umbilical [D). 



Ik the meantime three cross-connections have developed between the two vitelline veins 

 (Fig. 779, A, B, C), two of them passing ventral to the intestine and one dorsal to it, the 

 intestine thus becoming surrounded by two venous rings. The right half of the lower ring 

 and the left half of the upper one now degenerate {£>), and the persisting portions, which 

 terminate partly in the hepatic net-work and partly in the ductus venosus, become the portal 

 vein. The upper portion of the right umbilical vein has in the meantime made a connection 

 with the upper ring of the vitelline, and the part above the connection then degenerates, the 

 lower part becoming much reduced in size and persisting as a small parumbilical vein in the 

 anterior abdominal wall. This arrangement persists until birth, the j^lacental blood passing by 

 way of the left umbilical vein partly to the ductus venosus and thence by the inferior vena cava to 

 the heart and partly through the hepatic net-work byway of the communication between the left 

 umbilical and vitelline veins. At birth the placental supply of blood is of course cut off, the ductus 



DC 



DC 



Diagrams illustrating transformations of vitelline and 

 umbilical veins during development of liver-veins. DC. ducts 

 of Cuvier; /i, heart; 7'u, In, right and left umbilical veins; 

 rv. /z/, right and left vitelline veins ; rfz/, ductus venosus ; /)v, 

 portal vein. {Hochstetter.) 



