THE VASCULAR SYSTEM 277 



right side, however, forms the right subclavian artery while that of the left 

 side becomes the arch of the aorta. Since this is connected directly with 

 the left ventricle it carries only aerated blood to the dorsal aorta. The 

 aortic arches in man resemble those of other mammals. (Fig. 250) 



The Evolution of Arteries. Aside from the transformation of the 

 aortic arches, the phylogenetic changes in the arterial portion of the vascu- 

 lar system have not been profound. The main trunks persist throughout 

 the series (Fig. 248). The appearance of the subclavians and iliacs is 

 correlated with that of the paired fins, but once invented these vessels 

 persist in man to supply the arms and legs. In the Dipnoi pulmonary 

 arteries make their debut as branches of the sixth pair of aortic arches 

 when the lungs which they supply emerge from air bladders. The 

 connexion of the sixth aortic arch with lungs persists throughout 

 the vertebrate series. With the substitution of a metanephros for the 

 mesonephros of lower vertebrates new renal arteries are formed. The 

 history of the caudal artery is one of degeneration until in man it becomes 

 the rudimentary median sacral artery. The number of splanchnic 

 arteries supplying the intestine in man remains the same — three — as in 

 elasmobranchs. 



The Evolution of Veins. The phylogenetic alterations of the veins 

 are much more radical than those of the arteries just described. Few 

 veins persist throughout the entire chordate series. Included among 

 such persistent veins are the jugulars, the precava (the right common 

 cardinal or right ductus Cuvieri) and subintestinal, a part of the portal 

 vein. The usual assumption that the postcardinals persist as the azygos 

 and hemiazygos, except in part, is not supported by the evidence from 

 embryology. 



The primary veins are paired, e.g., the precardinals, postcardinals, 

 lateral abdominals or umbilicals, vitelUnes, and even the subintestinal 

 vein which is paired at the time of its first appearance in vertebrates. 

 The portal vein is one of the primitive veins of Amphioxus and the modi- 

 fications of its development in connexion with the right vitelline vein 

 appear to be recent adaptations. The postcava appears as a new vein 

 in Dipnoi and undergoes considerable reconstruction in mammals through 

 the addition of parts of the subcardinal anastomosis, right postcardinal, 

 and supracardinal veins. The views of investigators in regard to the 

 origin of the subcardinal veins are divergent. The evidence seems to 

 support the opinion that the subcardinals make their appearance with 

 that of the renal portal veins and that they persist throughout the verte- 

 brate series and in part are incorporated in the postcava of mammals. 

 In man, and to a lesser degree in other mammals, veins become differenti- 

 ated into a superficial set which drain the skin and outer organs, and a 

 deep set which carry blood away from the deeper organs of the body. 



