DEVELOPMENT OF THE ARTERIES 



263 



ventral aortae cranial to the third arch persist as parts of the internal 

 and external carotid arteries respectively. The third arches form the 

 stems 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 pul- 

 monary trunks, so that the aorta opens into the left ventricle and 

 the pulmonary trunk into the right ventricle. The descending aortae 

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

 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 



External carotid 



Innominate 

 artery 



Right sub- 

 davian artery 



Right pul- 

 monary artery 



Trunk of pul- 

 monary artery 



Internal carotia 

 Common carotid 



Aortic arch 



Ductus arteriosus 

 Vertebral artery 

 Subclavian artery 



Left pulmonary artery 

 Ventral aorta 



FIG. 273. Diagram showing the aortic arches and their derivatives in human embryos. 



intersegmental artery and forms part of the right subclavian artery, which 

 is thus a more complex vessel than the left. The segment of the fourth 

 arch proximal to the right common carotid becomes the innominate 

 artery. The fifth arches of amniotes are rudimentary (p. 100). 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 

 proximal portion of the right sixth arch forms the stem of the right pul- 

 monary artery, but the proximal portion of the left arch is incorporated in 

 the pulmonary trunk. Most of the pulmonary artery arises from a post- 

 branchial plexus; union with the sixth arch is acquired secondarily 

 (Huntington, 1919). 



The aortic arches of the embryo are of especial importance comparatively. Five 

 arches are formed in connection with the gills of adult fishes. In adult tailed amphibia, 



