Mammalia: Visceral Systems 



627 



arterial circle 

 (of Willisj 



subclavian a 

 internal 

 mammary 



dorsal aorta 



Fig. 477. Diagrams illustrating the changes which occur in the aortic arches of 

 mammalian embryos. (A) Ground plan of complete set of aortic arches. (B) Early 

 stage in modification of arches. (C) Derivatives of aortic arches, (br. ceph.) 

 Brachiocephalic artery; (cer. a.) cerebral artery; (lin.) lingual artery; (max.) 

 maxillary artery; (ophth. a.) ophthalmic artery; (stap. a.) stapedial artery; (thy.) 

 thyroid artery. (Adapted from several sources. Courtesy, Patten: "Embryology 

 of the Pig," Philadelphia, The Blakiston Company.) 



and left aortic trunks cross one another, each opening into the ventricle 

 of the opposite side. This extraordinary arrangement is virtually 

 repeated in birds in that the persisting right aorta emerges from the 

 left ventricle. In mammals only two arterial trunks, one from each 

 ventricle, emerge from the heart, in contrast to the three in reptiles. 

 Whereas in reptiles the right and left trunks of the fourth aortic arch 

 connect, respectively, with the left and right ventricles, in mammals 

 both the right and left members of the fourth arch connect with the 

 left ventricle. There is at no time a crossing of the two trunks. 



Omission of the left aortic trunk in birds is physiologically intel- 

 ligible. To the cold-blooded reptile, the left trunk, pouring blood from 



