300 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



(Sphenodon, turtles, squamata) or completely (crocodiles) separating the two 

 ventricles. The peculiar relations of the aortic arches have been mentioned (p. 

 283). Correlated with the differences between the aortic (fourth) arches of the 

 two sides in the majority of reptiles are certain features in the origin of the arteries. 

 Thus both of the subclavian arteries (lacking in snakes) arise from the right radix, 

 while the left gives rise to the coeliac artery. In many reptiles the anterior parts of 

 the postcardinals are replaced by vertebral veins. The renal portal system is 

 developed in the embryo and persists (much as in the amphibia) to a greater or less 

 extent in the adult. Usually paired 'anterior abdominal veins are present. 



BIRDS. The peculiarities of the heart and aortic arches were mentioned on 

 page 283. Birds have the same reduction of the postcardinals as is found in reptiles. 

 The renal portal system is formed in the embryo, but the only blood received by 

 the adult kidney comes through renal arteries like those of mammals. The iliac 

 veins extend to the postcava and lose all connexion with the anterior abdominal 

 veins. The paired epigastric veins persist only in front. 



MAMMALS. In the mammals the four chambers of the heart are completely 

 separated and the sinus venosus has been completely merged in the right atriumt 

 The persistent left fourth aortic arch forms the sole connexion between the hear. 



FlG. 306. Modifications of the origin of the carotid and subclavian arteries in 



mammals. 



and the dorsal aorta and from it arise the carotid and subclavian arteries, the 

 arrangement of these representing almost every possible condition (fig. 306). In 

 the lower groups (e.g., rodents) both Cuvierian ducts persist, but in the higher orders 

 a cross connexion (the innominate vein) arises between the trunks formed from 

 the jugulars and subclavian veins of the two sides (fig. 308) so that the blood from 

 the left side of the head, neck and fore limb joins that of the left side in a common 

 trunk, the precava (anterior vena cava) which enters the right atrium. With 

 this development the left Cuvierian duct, as such, disappears. 



The renal portal system has but a transitory existence in the embryo (best 

 developed in the monotremes) and early disappears with the degeneration of the 

 Wolffian bodies (mesonephroi). As these organs disappear a part of the capillary 

 system of the Wolffian bodies enlarges and forms a main trunk connecting the 

 postcava with the posterior parts of the postcardinal veins (fig. 307, C) which bring 

 the blood from the tail, the iliacs and the permanent kidneys. With farther develop- 

 ment (D, E) the left postcardinal is largely lost (except the part connecting with the 

 suprarenal and gonad of that side) and all the blood from the posterior part of the 

 body is returned by the right postcardinal and the postcava, which appear (fig. 

 308, A} as if they arose from a union of the iliac veins. Correlated with these 

 changes in the venous system and the impossibility of venous blood entering the 

 excretory organs, there is developed a renal artery from the aorta for each of the 

 permanent kidneys. 



