Mammalia: Visceral System? 



631 



pc~ 



Zha 



ney at all, but connect directly into the hind end of the postcava (Fig 



71E). Only arterial blood enters the 



kidneys. 



In sonic mammals, but not char- 

 acteristic of the ("lass as a w hole, I here 

 is a striking modification of the em- 

 bryonic venous system. In all verte- 

 brates the embryo develops a right 

 and a left precaval vein (duct of 

 Cuvier), each formed by union of the 

 postcardinal, precardinal (jugu- 

 lar), and subclavian of its side of the 

 body, and both opening into the right 

 auricle. In later embryonic stages of 

 many mammals a large vein develops, 

 extending transversely and just ante- 

 rior to the heart, connecting the left 

 precava, or the posterior end of the 

 left precardinal (jugular), with the 

 right precava. The result is that some 

 blood from the left side flows across 

 into the right precava (Fig. 479, left). 

 In some mammals (sloths, anteaters 

 and other "edentates"; whales; sea 

 cows; carnivores; monkeys, apes, 

 man), after the establishment of this 

 connection, that part of the left pre- 

 cava intervening between the trans- 

 verse vein and the heart eventually 

 undergoes complete atrophy so that 

 all the blood carried by its former 

 tributaries flows across into the right 

 precava (Figs. TIE, 479, right). In the 

 Australian duckbill and spiny anteater 

 (both egg-laying mammals), in marsu- 

 pials, and, with some exceptions, in 

 insectivores, rodents, bats, and ungu- 

 lates, both precaval veins persist al- 

 though in some cases the cross connec- 

 tion between them is established. 



The absence of the left precava in the adult mammal results in an 

 asymmetry of the main venous trunks equaling that of the aortic 

 trunks. A certain balance, or convenient spacing of the large vessels 



Fig. 479. Late embryonic stages 

 in the development of the veins of a 

 mammal. Ventral views. (Left) 

 Right and left postcardinals and 

 precardinals (ri) symmetrically 

 developed and with a cross con- 

 nection established between cor- 

 responding right and left veins. 

 (Right) Later stage. Reduction of 

 left postcardinal; obliteration of 

 left Cuvierian duct leaving left 

 jugular and subclavian connected 

 only with the right Cuvierian duct 

 (precava). (az) Azygos major; (c) 

 coronary vein; (ca) caudal vein; 

 (ei) external iliac; (g) gonads; (ge) 

 genital (spermatic, ovarian) vein; 

 (ha) hemiazygos; (i) ischiadic; (j) 

 jugular; (li) left innominate; (mtn) 

 metanephroi; (p) postcava; (pc) 

 postcardinal; (pr) precava; (r) 

 renal; (ri) right innominate; (s) 

 subclavian; (sic) superior inter- 

 costal. Postcaval elements are 

 crosslined; subcardinal, dotted; 

 other veins black. (Courtesy, Kings- 

 ley: '"Comparative Anatomy of 

 Vertebrates," Philadelphia, The 

 Rlakiston Company.) 



