IOI4 A MANUAL OF ANATOMY 



inferior vena cava of adult life, and the part of the left subcardinal vein 

 above the great transverse pre-aortic anastomotic vessel forms the terminal 

 portion of the left suprarenal vein. The great transverse pre-aortic anas- 

 tomotic vessel persists, and forms the mesial part of the left renal vein of 

 adult life. 



Additional important changes take place in the course of development, 

 which will now be stated. 



Metamorphoses of Left Posterior Cardinal Vein. — (i) The part below the left 

 extremity of the transverse iliac vein persists, and forms from above down- 

 wards: (fl) the commencement of the left common iliac vein, and {b) the left 

 i'fiternal iliac vein, into which latter the left external iliac vein subsequently 

 opens. 



2. The part between the left extremity of the transverse iliac vein and 

 the place of entrance of the left spermatic vein undergoes atrophy. 



3. The part between the place of entrance of the left spermatic (or ovarian) 

 vein and the future left renal vein persists, and forms the upper, or terminal, 

 part of the left spermatic (or ovarian) vein. This circumstance explains why 

 the left spermatic (or ovarian) vein opens into the left renal vein, whereas 

 the right spermatic vein opens into the inferior vena cava. 



4. The part immediaiiely above the left renal vein disappears. 



5. The remainder of the left posterior cardinal vein persists, and gives rise 

 to (a) the lower left azygos vein, {b) the upper left azygos vein, and (c) the 

 lower portion of the left superior intercostal vein, the upper portion of which 

 is formed by that part of the left anterior cardinal vein which lies immediately 

 below the left extremity of the transverse jugular vein. 



6. The part between the lower and upper left azygos veins, and the part 

 between the upper left azygos and left superior intercostal veins may, or may 

 not, undergo obliteration. 



Metamorphoses of the Right Posterior Cardinal Vein. — (i) The part below 

 the right extremity of the transverse iliac vein persists, and forms from above 

 downwards {a) the right common iliac vein, and (6) the right internal iliac 

 vein, into which latter the right external iliac vein subsequently opens. 



2. The part between the right extremity of the transverse iliac vein and 

 the future right renal vein persists, and forms the lower, or postrenal, division 

 of the inferior vena cava. Into this part, superiorly, the right spermatic 

 vein opens, which explains the mode of ending of that vein in the adult, as 

 compared with the mode of ending of the left spermatic vein, which, as stated, 

 opens into the left renal vein. 



3. The part immediately above the right renal vein disappears. 



4. The remainder of the right posterior cardinal vein persists, and gives 

 rise to the right azygos vein, which, superiorly, takes up the right superior 

 intercostal vein. 



The right superior intercostal vein is developed from the anastomotic 

 channels which connect the upper three thoracic segmental veins of the right 

 side. 



The right and left posterior cardinal veins, in the thoracic region, are con- 

 nected by two transverse retro-aortic anastomotic vessels. The lower of these 

 cross- branches persists, and forms the lower transverse azygos vein, which 

 diverts the blood from the lower left azygos vein into the right azygos vein. 

 The upper cross-branch also persists, and gives rise to the upper transverse 

 azygos vein, which diverts the blood from the upper left azygos vein likewise 

 into the right azygos vein. There may only be one transverse azygos vein. 



Metamorphoses of the Subcardinal Veins — Right Subcardinal. — (i) The 

 right subcardinal vein, as stated, becomes connected with the hepatic veins 

 by a fusion between hepatic and subcardinal capillaries dorsal to the liver 

 and within the caval mesentery. (2) The portion of it which lies above the 

 large transverse anastomotic vessel, connecting the subcardinal and posterior 

 cardinal veins below the level of the root of the superior mesenteric artery, 

 persists, and forms part of the prerenal division of the inferior vena cava. 

 (3) The portion of it on the caudal side of the large transverse anastomotic 

 vessel disai^pears. 



