TTTE VASCULAR SYSTEM 36 1 



three aortic arches persist in the adult, 3rd, 4th, and 6th as in reptiles. 

 The systemic arch of the 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. 299) 



The Evolution of Arteries. Aside from the transformation of the 

 aortic arches the phylogenetic changes in the arterial portion of the 

 vascular system have not been profound. The main trunks persist 

 throughout the series proving that nature holds fast to that which is good. 

 The appearance of the subclavians and iliacs is correlated with that of the 

 paired fins, but once invented these vessels persist in man to nourish 

 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 pulmonary 

 arteries with lungs persists throughout the vertebrate series. With the 

 substitution of a definitive kidney for the mesonephroi 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. (Fig. 297) 



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 

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

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

 tal 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 umbiHcals, 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- 



