THE MAMMALIA 



295 



the dorsal aorta. The subsequent fate of these arches is 

 shown in fig. 73. The truncus arteriosus bifurcates in front 



'.. 



jnd 



Fig- 73 



Diagram illustrating the derivation of the arterial trunks c f the 

 adult from the embryonic aortic arches in a mammal. The tran- 

 sient embryonic vessels are drawn in outline. md, mandibular 

 arch ; hy, hyoid arch ; l-IV, the four branchial arches, ao, the 

 aortic arch formed from the second branchial arch of the left side; 

 6r, brachial artery ; ce, external carotid ; ', internal carotid ; rf.a, 

 ductus arteriosus, connecting the pulmonary and aortic arches ; 

 d.ao, dorsal aorta ; put, pulmonary artery ; sea, right subclavian 

 artery, formed by the upper part of the second branchial arch of 

 the right side ; vr^ vertebral artery. 



of the second branchial arch, and each bifurcation is continued 

 forwards as the external carotid artery. The internal carotids 



