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. 
Fic. 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 (¢.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. 
