THE DEVELOPMENT OF THE VASCULAR SYSTEM 



245 



third and fourth arches disappears. The fourth aortic arch on the left side 

 enlarges and becomes the arch of the aorta (arcus aorta) which is then continued 

 caudally through the left dorsal aortic root into the dorsal aorta. On the right 

 side, the fourth arch becomes the proximal part of the subclavian artery. Since 

 the third, fourth, fifth, and sixth arches really leave the ventral aortic trunk as a 

 single vessel, it will be seen that these changes bring it about that the common 

 carotid and subclavian on the right side arise by a common stem, the innomi- 

 nate artery, which in turn is a branch of the arch of the aorta. On the left side, 



Common carotid arteries 



Int. carotid artery (right) 

 Ext. carotid artery (right) 



Int. carotid III 



Subclavian IV 



V 



VI 



Innominate artery 



Subclavian artery (right) 



Int. carotid artery (left) 

 Ext. carotid artery (left) 



III Int. carotid 



IV Arch of aorta 

 V 



VI Ductus arteriosus 



Pulmonary artery 

 Subclavian artery (left) 

 Aorta 



Fig. 220. — Diagram representing the changes in the aortic arches of a Mammal. 

 Compare with Fig. 219. Modified from Hochstetter. 



for the same reason, the common carotid is a branch of the arch of the aorta. 

 The fifth aortic arch from the beginning is rudimentary and disappears very 

 early. The sixth arch on each side undergoes wide changes. A branch from 

 each enters the corresponding lung. On the right side the portion of the sixth 

 arch between the branch which enters the lung and the dorsal aortic root disap- 

 pears as does also that portion of the right dorsal aortic root between the 

 subclavian artery and the original bifurcation of the dorsal aorta. On the 

 left side however, that portion of the sixth arch between the branch which 

 enters the lung and the dorsal aortic root persists until birth as the ductus 



