CIRCULATORY ORGANS. 289 



may remain distinct or it may fuse with the others at the base, and 

 then its independent portion appears as a branch of the common 

 iliac artery. 



The dorsal aorta, which continues into the tail, is called the caudal 

 artery behind the point where the sciatics (common iliacs) arise. 



A cutaneus artery, arising from either the subclavian or the 

 pulmonary artery of either side (both conditions occur in the amphibia), 

 runs backward in the skin of the trunk, and may extend back and unite 

 with the epigastric artery. When, as in the amphibia, these arise 

 from the pulmonary they contain venous blood and the skin acts as 

 a subsidiary respiratory organ (p. 258). 



VEINS. 



The position and development of the chief longitudinal venous 

 trunks have already been outlined. Both these and other veins yet 

 to be mentioned frequently undergo shiftings of position and other 

 modifications during growth, but before describing these changes some 

 other vessels must be described. 



With the development of the limbs corresponding veins arise (fig. 

 294), a subclavian vein for each fore limb, a common iliac for the 

 hind leg, these bringing the blood from the appendage to the trunk. 

 In the young each subclavian empties into the postcardinal of the same 

 side, but in the adult the opening may shift to the Cuvierian duct. 

 The common iliac vein likewise empties into a vein, the epigastric 

 or lateral abdominal, which runs forward in the body wall to connect 

 with either the postcardinal or the duct of Cuvier (fig. 294, A}. This 

 condition obtains throughout life in some elasmobranchs, but higher 

 in the scale the iliac vein, while retaining its connexion with the 

 epigastric, grows toward the middle line and joins the postcardinal of 

 the same side, a condition which is permanent in amphibia and reptiles 

 (fig. 294, B, C), where blood coming from the hind limb has two 

 routes to the heart. 



The epigastric veins of the two sides may fuse in the median line 

 in front (amphibia, some reptiles, birds), forming an anterior ab- 

 dominal vein (fig. 294, C) which reaches the heart by passing through 

 the remains of the ventral mesentery (ligamentum teres) to the liver 

 and thence forward. A similar anterior abdominal vein has been 

 described in Echidna but is unknown elsewhere in the mammals. 

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