AORTIC ARCHES OF BIRDS — GLENN Y 



551 



It is possible that th.e left systemic arch arose at the base of the left 

 innominate artery and continued to function as in reptiles. In some 

 forms, the left systemic arch may have become somewhat reduced. 

 Both ducti botalli were probably reduced to ligamentous vestiges, 

 although either may have presented a lumen from time to time. 



The subclavian, pectoral stem, and branches were possibl}^ variable, 

 but essentially similar to those of most present-day orders of birds. 

 The vertebrals and superficial cervicals either had a common root or 

 arose separately but in the same general location, near the thyroid 

 gland. 



The ventral carotids probably maintained their anterior cephalic 

 connections. The condition of uniform bilaterality probably persisted 

 in most of the earliest forms. It is possible that in these early avian 

 ancestral forms there may have been reduction in diameter of both 

 the left systemic arch and the left radix aortae. 



The ventricle was probably completely divided into left and right 

 chambers, and the pulmonary root emerged from the right ventricle 

 while the aortic root emerged from the left ventricle. One essential 

 difference exists between the crocodilian heart and the avian heart — 

 that is, the left systemic arch probably arose from the aortic root or 

 from the innominate artery instead of having a separate root arising 

 or emerging from the right ventricle as in the crocodilian heart. 



Subclass NEORNITHES 



Most of the ancestral forms were probably bicarotid, although some 

 may have been undergoing evolutionary changes toward the unicarotid 

 condition. For the most part, there probably was a great similarity 

 in the aortic arch system of the early Neornithes to that of the true 

 avian archaeornithial ancestors. Certain advances resulting from 

 both structural and functional modifications, including atrophy and 

 deletion of parts, undoubtedly took place. Among these early modi- 



EXPLANATION OF SYMBOLS ON FIGURES 113-118 



Ductus caroticus. 



Ductus shawi. 



Arteria gallinae (supplies trachea). 



Innominate arteries. 



Ligamentum aortae. 



Ligamentum botalli. 



Oesophageal arteries. 



Pectoral arteries. 



Radix (radices) aortae. 



Subclavian artery. 



Subscapular artery. 



Thoracic or intercostal artery. 



Vertebral artery. 



