THE MAMMALIA 295 



the dorsal aorta. The subsequent fate of these arches is 

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



Fig- 73 



Diagram illustrating the derivation of the arterial trunks of the 

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

 sient embryonic vessels are drawn in outline. tjid^ mandibular 

 arch; hy, hyoid arch; /-//', the four branchial arches, at?, the 

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

 br^ brachial artery; ce^ external carotid ; ci^ internal carotid ; d.a, 

 ductus arteriosus, connecting the pulmonary and aortic arches ; 

 d.ao^ dorsal aorta ; ^«/, pulmonary artery; sca.^ right subclavian 

 artery, formed hy 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 



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