322 



Veterinary Obstetrics 



The aortse continue forward beyond the aortic arches and run 

 alongside the brain, to constitute the internal carotid arteries. 



Changes now begin to appear, looking toward the foundation 

 of the adult plan of circulation. The middle portions of the 

 aortic arches in the mandibular and hyoidean arches disappear ; 

 the connection between the truncus arteriosus and the aortae, 

 through these arches, ceases to exist and the proximal ends of 

 these vessels remain as the external carotid arteries. 



The third aortic arch remains and retains its connection with 

 the anterior portion of the aorta and its proximal end or stem 

 constitutes the common carotid artery. The connection between 

 the portion of the aorta posterior to the junction with the third 

 aortic arch and anterior to the fourth disappears, thus separating 

 the carotid system from the aortic vessels. 



Common carotid 



Recurrent laryngeal \ 



nerve {right) 

 Right subclavian 



Innominate artery 

 Ascending aorta 



Vagus nerve 

 External carotid 



Internal carotid 



Vertebral artery 

 Arch of aorta 

 Left subclavian 

 Ductus arteriosus 



Pulmonary trunk 



Fig. 50. Diagram illustrating tlie fate of the aortic arches. 

 (Modified from Heisler. ) 



The left fourth aortic arch persists, to eventually become the 

 aorta, while the fifth, or, according to those authors who re- 

 cognize six aortic arches, the sixth, arch ultimately becomes the 

 pulmonary artery. See Pigs. 49 and 50. 



