AORTIC ARCHES OF BIRDS — GLENNY 609 



All birds, insofar as presently known, develop a complete aortic 

 arch system, and this system undergoes a series of developmental 

 (atrophic) deletions and other modifications which result in the adult 

 arterial arrangement-patterns, most of which have been set forth in 

 the previous chapter. It is now a question as to whether the fourth 

 and fifth aortic arches make an appearance in at least several of the 

 TrochLlidae, and, if they do, for what period of time they are present. 

 Glenny (1953b, 1954c) has shown that the right systemic arch may be 

 absent in several species of hummingbirds, and that the ductus 

 caroticus serves the function of the fourth arch in these instances. 



That widely separated orders of birds should undergo similar evo- 

 lutionary changes in the aortic arch system is in itself a fact worthy 

 of note, but that a somewhat similar instance of deletion of the dorsal 

 carotid should occur in Alligator mississippiensis is of still greater 

 interest (Reese, 1914), especially in view of the fact that some zooJ- 

 ogists have linked the CrocodUia and Aves to the same basic reptUian 

 stem (Archosauria) . In the alligator, the left dorsal carotid remains 

 as the functional vessel and, as in most birds, this vessel enters the 

 hypapophysial canal and passes anteriorly to the head where it 

 sends off left and right branches which then further divide and supply 

 the various parts of the cranial region. Thus the carotid arrangement 

 in the alligator would be referred to as laevocarotidinae, and, as has 

 aheady been demonstrated, this arrangement is quite common among 

 the present-day bu-ds. 



Within the class Aves there is abundant evidence to show that 

 evolution of the aortic arch system is stiU m progress, and that cul- 

 mination of this evolution may be the complete loss of the dorsal 

 carotids anterior to the carotid arch. In such instances where this 

 condition has been found, the vertebrals and the superficial cervicals 

 take over the function of supplying blood to the head. 



Reduction and loss of the dorsal carotids may take place either 

 by a series of microdeletions or by macrodeletion. Two known 

 instances of the latter arc found in Bucorvus abyssinicus and Rhopo- 

 dytes viridirostris , while a macrodeletion of but one dorsal carotid is 

 known to occur in Rhamphococcyx curvirostris erythrognathus. Such 

 unilateral macrodeletions may be of more frequent occurrence than 

 is known at present, and further studies of the aortic arch system in 

 birds may reveal still other instances of bilateral atrophy and deletion 

 of the dorsal carotids. These instances of carotid atrophy have 

 been found to occur in the Coraciiformes and the Cuculiformes. 

 The macrodeletions mentioned above were found in normally bicarotid 

 (A-1) forms. 



Evolution of the aortic arch system from a bicarotid to a uni- 

 carotid condition is of interest when considering the implications of 

 paramorphogenesis. Presumably, for the most part, the two dorsal 

 carotids become conjugate vessels. This is followed by reduction 



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