DEVELOPMENT OF THE ARTERIES 



263 



dorsal and ventral aortas cranial to the third arch persist as parts of the internal 

 and external carotid arteries respectively. The third arches form the steins of 

 the internal carotids, while the ventral aortae between the third and fourth arches 

 become the common carotids. In embryos of 15 mm. the bulbus cordis has been 

 divided into the aortic and pulmonary trunks, so that the aorta opens into the 

 left ventricle and the pulmonary trunk into the right ventricle. The dorsal 

 aortae between the third and fourth arches disappear, but the fourth arch on the 



Aortic arch y 

 Aortic arch 2 



Aortic arch i 



Dorsal aorta 



Aortic arch 4 

 A ortic arch 6 



Esophagus 

 Trachea 



Ventral aorta 



Pulmonary artery 

 Bulbus cordis 



B 



Aortic arch 3 



Int. carotid artery. 

 Aortic arch 2 



External carotid 

 Aortic arch 6 

 Pulmonary artery. 



Bulbus cordis 



Aortic arch 4 



Aortic arch 5 



Dorsal aorta 



Fig. 272. — Aortic arches of human embryos: A,oi5 mm.; B, of 7 mm. (after Tandler). I-IV, pharyn- 

 geal pouches. 



left side persists as the aortic arch of the adult. On the right side, the fourth aortic 

 arch persists with the descending aorta as far as the seventh intersegmental artery 

 and forms part of the right subclavian artery, which is thus longer than the left. 

 The segment of the fourth arch proximal to the right common carotid becomes the 

 innominate artery. On the right side, the sixth arch between the origin of the 

 right pulmonary artery and descending aorta is early lost; on the left side, it 

 persists as the ductus arteriosus and its lumen is only obliterated after birth. The 



