Mammalia: Visceral Systems 625 



blood of much better quality than that in the left. In view of this fact, 

 it is significant that the arteries to the head (carotids) come off from 

 the right aortic trunk. Also, the arteries (subclavians) to the forelegs 

 receive blood from the right aorta. In lizards right and left subclavians 

 come off together from the dorsal region of the right aorta. In croco- 

 dilians and chelonians the subclavians arise in conjunction with the 

 carotids from a common trunk (innominate), which comes off from 

 the right aorta ventrally and close to the heart. In crocodilians, whose 

 ventricular septum is complete, the right aortic trunk carries quite 

 pure aerated blood. But in all modern reptiles (including crocodilians) 

 the left aortic trunk, usually of less diameter than the right, carries 

 blood from the right side of the ventricle into the dorsal aorta and thus 

 more or less vitiates the blood delivered by the aorta to the trunk and 

 tail. 



The embryonic development of a bird's heart is substantially 

 like that of the reptiles. At an early stage, the fourth aortic arch is 

 complete on both right and left sides, as it is permanently in reptiles, 

 but, before the differentiation of the trunks at the apex of the ven- 

 tricular region has been completed, the left aortic trunk suffers 

 complete degeneration. (It has been reported that in rare cases the 

 complete reptilian arrangement is temporarily present in the bird 

 embryo.) Meanwhile, the subclavian and carotid connections are 

 established, as in chelonians and crocodilians, with the right aortic 

 trunk, which has become walled off so as to serve as the only outlet 

 from the left ventricle. The left aortic arch having been obliterated, 

 the pulmonary trunk, derived from the embryonic arch correspond- 

 ing to the sixth of a fish, remains as the only outlet from the right 

 ventricle. 



In the mammalian embryo six aortic arches may be present, 

 although not all at the same time (Figs. 474, 477). The more anterior 

 arches begin to degenerate before the more posterior are fully devel- 

 oped. The fifth arch is rudimentary or, in some cases, not to be found 

 at all. As in reptiles and birds, the permanent connection between the 



Fig. 476.— {Continued) 

 of the subclavian arteries (sc) in (C) is questionable.] (asc) Anterior subclavian: 

 (d) ductus Botalli; (da) dorsal aorta; (ec) external carotid; (ic) internal carotid; 

 (la) left auricle; (Isa) left systemic arch; (Iv) left ventricle; (p) pulmonary artery; 

 (pv) pulmonary vein; (r) portion of lateral aorta remaining open only in Sphenodon 

 and some Lacertilia as ductus arteriosus; (ra) right auricle; (rsa) right systemic 

 arch; (rv) right ventricle; (sc) subclavian; (sp) septum; (spa) interauricular septum; 

 (spv) interventricular septum; (so) sinus venosus; (o) ventricle; (vc) vena cava 

 superior; (vci) vena cava inferior. Arrows from sinus venosus indicate main stream 

 of venous blood; arrows with dotted line from left auricles indicate stream of 

 arterial blood. (1-6) Original series of six embryonic aortic arches. (From Good- 

 rich: "Studies on the Structure and Development of Vertebrates." By permission 

 of The Macmillan Company, publishers.) 



