external carotid 



\i ^internal carotid 



ductus caroticus (when open) 



common carotids 



\J I brachiocepholic (innominate) 



left and right 



brachiocephalic 



fused 



MAMMAL 



HUMAN C CAT Q HEDGEHOG E OX 



ORNIiHORHYNCHUS KANGAROO 

 WOMBAT 



Figure 11-10. Variations in the arterial roots of mammals. (After Romer, 1955, and Owen, 1868} 



odile, but this has not been observed. The loss of this arch 

 results in the functional separation of pulmonary and sys- 

 temic circulations in the bird, a parallelism to that observed 

 in the mammal. 



As a result of the elongation of the neck and the posterior 

 displacement of the heart, relationships between the carotids 

 are altered. In the foetus of 6 to 7 days, a commissure con- 

 nects the external and internal carotids near the head, and 

 the external carotid stem may lose connection with its 

 cranial extreme. The stem is now called the ascending 

 esophageal or superficial cervical. 



The internal carotids come to lie next to each other at 

 the midline in a hypophyseal canal. In some the carotids 



fuse, and the left or right root for this common vessel may 

 be the larger. In other species, only the left carotid is re- 

 tained. The variations of the carotids were used by Garrod 

 (1873) in the classification of birds. The changes observed 

 in the bird parallel those in the crocodilian in which only 

 the left carotid is retained and the external carotid stems 

 (collateralis colli) maintain their distal connections. 



The posterior displacement of the heart is also reflected 

 in the subclavian arteries. The subclavian arises as a seg- 

 mental artery from the dorsal aorta posterior to the pharyn- 

 geal region. About the sixth day the heart is moved back 

 in relation to the limb; a new subclavian stem develops 

 from the external carotid region (ventral radix aortae) of 



carotid 



systemic arcl 



subclavian (pri 



internal carotid 



external carotid 



Imona 



subclavian (secondary) 



A IGUANA B VARANUS C CROCODILE D BIRD 



Figure 11-11. Aortic arches of reptiles and the bird. (B, C, D after Goodrich, 1930) 



348 • THE CIRCULATORY SYSTEM 



