VENOUS SYSTEM. 291 



hinder extremity (Fig. 232, lie, Hy, CV). In some cases the internal 

 and external iliacs come off separately from the aorta. The 

 function of the external iliac may be largely taken by a sciatic 

 artery arising separately from the aorta or iliac artery in the 

 pelvic region (Birds, and to a less extent in Reptiles) (comp. 

 Fig. 233, Is). 



The main vessels branch out in the limb in a manner 

 essentially similar to that already described for the anterior 

 extremity. 



VENOUS SYSTEM 



The numerous phases of development through which the 

 venous system passes are very instructive. 



The cardinal veins, 1 which open into the ductus Cuvieri, have 

 been already described, and it is only necessary to add that in 

 Fishes there is a renal-portal system interposed between the 

 posterior cardinals and the caudal vein, the latter dividing up into 

 capillaries in the kidney. The vessels returning the venous blood 

 from the alimentary canal, the pancreas, and the spleen, also 

 divide up into capillaries in the liver (hepatic portal system). 

 The blood passes out from the latter by the hepatic veins into the 

 postcaval, whence it is conducted into the right auricle. 2 



From the Amphibia onwards the postcaval becomes of in- 

 creasing importance ; it receives the blood from the kidneys and 

 generative organs, as well as that from, the posterior extremities, 

 pelvis, and body-walls. The posterior cardinals decrease propor- 

 tionately, and become to a certain extent lost. 



The unpaired anterior abdominal (allantoic) vein plays 

 a great part in Reptiles and Amphibians : it arises mainly from 

 the crural vein of either side, and to a lesser degree also from 

 the veins of the urinary bladder ; it then passes along the ventral 

 body-walls to the liver, where it anastomoses with the portal 

 system, a small branch sometimes (c.g. Frog) going to the heart. 

 In Birds and Mammals the allantoic vein has an important function 

 in the embryo, but becomes obliterated on the atrophy of the 

 allantois at the close of foetal life : it seems, however, to be retained 

 throughout life in Echidna (Beddard). 



From Birds onwards a renal-portal system no longer appears ; 

 but a hepatic portal system persists in all the higher Vertebrates. 



1 It is interesting to note the almost lacvmar condition of the veins in many 

 Elasmobranchs : the two posterior cardinals, for instance, unite in the middle of 

 the body-cavity to form a large sinus, with which others bringing back the blood 

 from the generative organs are connected. 



2 In the Skate a large lateral vein lies along the outer side of the body-cavity 

 on either side, and receives the blood from the pectoral fins, abdominal walls, and 

 iliac region. A similar vein has been found in embryos of other Elasmobranchs 

 (c.g. Scymnus, Acanthias, Mnstelus), and it seems highly probable that it corresponds 

 to the vein of the primitive lateral fm-folds (T. J. Parker) (comp. pp. 85 and 86). 



u 2 



