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MR. C. H, O DONOGHUE ON THE 



These arches soon become reduced to three on each side, viz. 

 3, 4, and 6, bj the disappearance of arches 1, 2, and 5. Of the 

 remaining arches, 3 is the carotid, 4 the systemic, and 6 the 

 puhnonary. By the separation of the trnncus arteriosus into 

 three tvibes the two carotids and the right systemic have a common 

 opening into the ventricle ; the left systemic opens separately, 

 and the two pulmonaries open by a common vessel (text-fig. 88). 

 The most remarkable change in the development is the enormous 

 lengthening of the carotids, brought about partly by the elonga- 

 tion of the neck but largely by the caudal shifting of the heart. 

 Thus it happens that in the adult condition the 3rd arch is far 

 removed from the 4th and 6th arches. 



Text-fiff. 89. 



Diagram to show changes in embryonic arterial arches. It shows the change 

 from the condition in text-fig. 88 to the definitive state. The filled-in portions 

 represent the vessels left in the adult, and those indicated by dotted lines the 

 vessels that disappear in the course of development. 



A.B. Basilar artery. C.C. Left common carotid. 

 Th.A. Thyroid. Other letters as in text-fig. 88. 



Adapted from Hochstetter (22). 



P.C. Primary carotid. 



The Carotid Arch goes through considerable changes in the 

 course of its development. The two common carotids arise from 

 the systemic by one root, the primary carotid (carotis primaria 



