BRANCHIAL ARTERIES 6oi 



Internal carotid : anterior prolongation of dorsal aorta, plus 



third arch. 

 External carotid : anterior prolongation of ventral aorta. 

 Common carotid : ventral aorta between third and fourth 



arches. 



Carotid gland or labyrinth : remains of capillaries of second gill. 



Carotid duct ( = ductus Botalli) : dorsal aorta between third and 



fourth arches. It persists in the adults of Apoda, some 



Urodela (e.g. Triton), and some Reptilia (e.g. Sphenodon, 



Lacerta). 



Systemic arch : basal portion of ventral aorta, plus fourth 



arch, plus lateral dorsal aorta. 

 Fifth arch : persists in adult Urodela. 



Ductus Botalli (= ductus arteriosus) : dorsal portion of sixth 

 epibranchial. It persists in Apoda, Urodela, and many 

 Reptilia (e.g. Sphenodon, Alligator, some Chelonia). In 

 Anura and in other amniotes, including mammals, it is 

 represented by a fibrous strand. 

 Pulmonary : a spht-off part of the ventral aorta, plus the ventral 



portion of the sixth arch. 

 Further development has taken place along two main lines. 

 In the theropsidan type, which has given rise to the mammals, 

 the ventral aorta becomes spirally spUt to its base, so that the 

 pulmonary trunk comes from the right side of the heart, the 

 aortic trunk (which forms systemics and carotids) from the left. 

 At the same time the ventricle becomes divided into right and 

 left halves, so that the pulmonary contains only deoxygenated 

 blood and the aorta only oxygenated. The posterior portion 

 of the right lateral dorsal aorta distal to the subclavian (i.e. 

 the right systemic) atrophies, so that what is called the right 

 subclavian includes also a portion of the dorsal aorta. Neither 

 this condition nor anything approaching it is found in hving 

 reptiles, but presumably the extinct Therapsida, from which 

 the mammals are derived on grounds of skull structure, had 

 something intermediate between that of the mammals and that 

 of the Amphibia. The relationship of these two is shown m 



Fig. 465- u u- ^ 



In the culmination of the sauropsidan type m the birds a 



similar result has been reached by different means. Not onl\- 



has the pulmonary been separated from the aorta to come from 



the right side of the heart, but the aorta itself is split so that one 



M.Z. — 20 



