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TEXT-BOOK OF EMBRYOLOGY 



undergo changes; some disappear and others become portions of the large 

 arterial trunks which leave the heart. In connection with the following 

 description, constant reference to Figs. 219 and 220 will assist the student in 

 understanding the changes. 



The first and second arches soon atrophy and disappear. The third 

 arch on each side becomes the proximal part of the internal carotid artery, 

 while the continuation of the dorsal aortic root, cranially to the third arch, 

 becomes its more distal part. The continuation of the ventral aortic root 

 cranially to the third arch, becomes the proximal part of the external carotid 



Common carotid arteries 



Int. carotid artery (right) 

 Ext. carotid artery (right) 



n , 



Int. carotid III 



Subclavian IV 



V 



VI 



Innominate artery 



Subclavian artery (right) 



Int. carotid artery (left) 



Ext. carotid artery (left) 



II 



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. 



artery, while the portion of the ventral aortic root between the third and 

 fourth arches becomes the common carotid artery. The portion of the dorsal 

 aortic root between the 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 proxi- 

 mal 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 



