196 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 





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Fig. 207. Venous 

 trunks of man. a, azy- 

 gos; I, caudal (sacralis 

 media); ci, common 

 iliac ; ei, external iliac ; 

 ej, external jugular ; f, 

 femoral ; g, genital ; ha, 

 hemiazygos (azygos 

 minor); hy , hypo- 

 gastric; 77, internal 

 iliac ; 7/', internal jugu- 

 lar ; k, kidney ; li, left 

 innominate; /, pre- 

 cava ; ph, phrenic ; /(', 

 postcava ; r, renal ; ri, 

 right innominate; 

 sc, subclavian ; sr, supra- 

 renal ; si, superior in- 

 tercostal ; t, thyroid. 



portal blood returns to the heart by way of 

 the postcava, while the posterior cardinals 

 merely receive that which comes back from 

 the bod}' walls by way of the intersegmental 

 veins. 



In the amniotes, a renal portal system 

 never reaches that development seen in the 

 ichthyopsida, and is only found i-n connec- 

 tion with the Wolffian body, i.e., in embry- 

 onic life. When first formed, blood is 

 returned from the kidney by the posterior 

 cardinal veins ; and these extend back and 

 receive the iliac veins as well, no interrenal 

 vein being formed by the co-operation of 

 caudal and postcardinal \'eins. When the 

 postcava extends back as far as the per- 

 manent kidneys, it sends a renal vein to 

 each ; and from this point backwards it ab- 

 sorbs the right postcardinal, while on the 

 other side lateral veins extend out and take 

 the blood formerly brought to the postcar- 

 dinal of the left side by the intersegmental 

 veins. Fig. 208 C. 



Farther in front a transverse vein arises 

 from the right postcardinal, and crosses 

 over and unites with the left, which now 

 loses its connection with the ductus Cu- 

 vierii and becomes the hemiazygos vein, the 

 blood from which now passes across into 

 the right post cardinal, called in man the 

 azygos or azygos major. In this way all of 

 the blood from the hinder half of the body 

 {i.e., from the kidneys and behind) flows 

 back to the heart through the postcava. 

 The anterior portion of the left posterior 

 cardinal may retain its connection with the 

 anterior veins (left innominate) of the same 



side, and become converted into a superior intercostal vein. 



