COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 267 



The ventral aorta in mammals as in birds is split into two trunks, in contrast with the 

 three trunks common to reptiles. These two are the pulmonary artery and the aorta. The 

 pulmonary artery leaves the right ventricle, the aorta the left ventricle. All of the systemic 

 veins open into -the right auricle and the pulmonary veins into the left auricle. The right 

 half of the heart is venous, and the left arterial, and owing to the completion of the interven- 

 tricular septum there is no mixing of arterial and venous blood in the heart, but a complete mid 

 perfect double circulation is maintained through the heart. The venous blood passes from the 



Fig. 58. — Diagrams to show the evolution of the aortic arches. A, primitive condition with six 

 aortic arches. B, fishes, the first aortic arch missing. C, some urodeles, the first, second, and fifth 

 arches missing. D, anurans, with the connection k between the pulmonary arteries e and the aorta 

 obliterated. E, reptiles, showing the ventral aorta split into three trunks and the fourth aortic arch 

 h and i, persistent on both sides. F, birds, the ventral aorta split into two trunks, and the fourth 

 aortic arch h persistent on the right side only. G, mammals, the ventral aorta split into two trunks and 

 the fourth aortic arch i persistent on the left side only, a, interruption of the aortic arches by the gill 

 capillaries in fishes; b, internal carotid; c, external carotid; d, pulmonary, developed from the sixth 

 arch; e, ventral aorta; /, dorsal aorta; g, aortic arch; h, right fourth aortic arch, called the right aorta 

 above urodeles; i, left fourth aortic arch, called the left aorta above urodeles; j, common carotid; k, 

 arterial ligament or obliterated vessel originally connecting pulmonary and aorta; /, subclavian. 

 (Slightly modified from Wilder's History of the Human Body, courtesy of Henry Holt and Company.) 



right auricle into the right ventricle and out through the pulmonary artery to the lungs, where 

 it is aerated. The arterial blood returns by way of the pulmonary veins to the left auricle, 

 passes into the left ventricle, and out of the aorta. 



Little trace is left in the adult mammal of the original system of aortic arches passing 

 around the pharynx. As in all of the land vertebrates, the bases of the common carotid arteries 

 represent the remains of the third aortic arches; the arch of the aorta is the left fourth aortic 



