AORTIC ARCHES OF BIRDS — GLENNY 541 



mine, no specific study has been made of the origin and development 

 of this vessel and the other pectoral arteries. 



Anterior intercostal supply is derived from the ventrally located 

 vessels, variously named, that arise as branches of the subclavian 

 arteries. There are no segmentally arranged vessels arising from the 

 right posterior radix aorta as in mammals. Posteriorly, the inter- 

 costal muscles are supplied by segmentally arranged arteries which 

 arise as branches of the abdominal aorta. No connection with pos- 

 teriorly located arteries could be established, and it is presumed that 

 the so-called internal mammary is not homologous with that of mam- 

 mals but is an intercostal artery not homologous with the inter- 

 costals of mammals. 



The above observations were made possible by materials especially 

 prepared for this study by Ward's Natural Science Establishment. 

 Three-day chicks were doubly injected with colored plastic and the 

 entire birds were then treated with corrosive solutions. As a result of 

 this treatment, it was found that the left radix aortae could be in- 

 jected for about half of its normal length. 



Changes in arrangement of thoracic and cervical arteries 



In birds, several significant changes may take place during the 

 course of embryonic development of the individual aside from and in 

 addition to (1) loss of the first, second, and fifth aortic arches, (2) 

 loss or functional modification of the ductus caroticus, (3) loss of the 

 left fourth aortic arch, (4) atrophy or functional modification of the 

 left radix aortae, (5) atrophy or loss of the ductus botalli, (6) the 

 shunt anastomosis between the dorsal and ventral carotids (anterior 

 radices aortae) , and (7) the accompanying functional modification of 

 the posterior end of the ventral carotid into an ascending oesophageal 

 or superficial cervical artery and the anterior end of this same vessel 

 into a descending oesophageal or superficial cervical artery. 



The dorsal carotids usually migrate to a median ventral position 

 along the long axis of the cervical vertebrae and, with the 

 development of the ventral cervical musculature, soon become en- 

 closed within the hypapophysial canal. These vessels then follow the 

 course of this canal to a point near the site of articulation between 

 the third and fourth cervical vertebrae, where they emerge and send 

 off branches comparable to those which join the internal and external 

 carotid arteries. 



It should be noted that in most orders and families of birds the right 

 dorsal carotid artery comes to lie in a position dorsad to the left 

 dorsal carotid artery, within the hypapophysial canal. This par- 

 ticular orientation of the carotids may be attained as a result of the 

 growth of the ventral cervical muscles and their encroachment upon 



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