626 Comparative Morphology of Chordates 



heart and the dorsal aorta is by way of the fourth aortic arch. A 

 remnant of the third arch persists, forming the posterior region of the 

 carotid arteries. The dorsal region of the sixth arch degenerates, 

 leaving the ventral parts of the two vessels to become the pulmonary 

 arteries. 



In development of the mammalian heart, the primarily undivided 

 arterial trunk at the apex of the primarily undivided ventricle becomes 

 divided by a longitudinal septum so placed as to separate its lumen 

 into a ventral space into which open the right and left vessels of the 

 fourth aortic arch, and a dorsal space into which open the two 

 members of the sixth aortic arch. This septum then grows backward, 

 but twisting spirally to such a degree that the spaces on either side of 

 its more posterior region are right and left instead of dorsal and ventral 

 as they are anteriorly. The now median (sagittal) posterior edge of 

 this septum finally joins the anterior edge of the septum between right 

 and left ventricles, with the result that the anterior ventral space 

 (fourth arch) continues back as the posterior left space, while the 

 anterior dorsal space (sixth arch) continues back as the posterior 

 right space. Completion of the process of splitting the ventricle and 

 its aortic outlet leaves both right and left members of the fourth 

 aortic arch opening by a common trunk from the left ventricle, 

 while ventral remnants of the sixth arch, having made connections 

 with the lungs, join in a common trunk which is the only outlet of the 

 right ventricle. Each subclavian artery develops as a branch of the 

 aortic trunk of its own side. In the later embryo, that part of the right 

 arch intervening between the dorsal aorta and the right subclavian 

 completely disappears. A certain extent of the basal region of the 

 adult "subclavian" is therefore a persisting ventral remnant of the 

 embryonic right aortic trunk (Figs. 476, 177). Incidentally, it appears 

 that the mammalian subclavians, arising dorsally as branches from 

 the right and left trunks of the fourth arch, are not to be regarded as 

 homologous with subclavians which arise ventrally as branches of the 

 carotids, as happens in chelonians, crocodilians, and birds. 



Comparison of reptiles, birds, and mammals reveals two 

 radically different patterns in the arrangement of the main arteries 

 leading out from the heart and in the relations of these vessels to the 

 ventricular cavities. In adult animals of all three Classes there is a large 

 degree of asymmetry in the plan of these arteries (Figs. 73, 476). In 

 birds the pattern of the asymmetry is identical with that of modern 

 reptiles, with the important difference that the left aortic arch is only 

 temporarily present in birds. This, however, does not alter the pattern. 

 It is merely omission of one element of the pattern. In reptiles right 



