AORTIC ARCHES. 705 



left side the anterior root and neighboring part of the fifth arch 

 are transformed into the pulmonary artery ; the remaining part 

 of this arch continues as the ductus arteriosus, which connects the 

 pulmonary artery with the permanent aorta. 



The fourth left arch, in mammalia, becomes the permanent 

 aorta. At the junction of the fourth and fifth left posterior 

 roots the left subclavian artery is given off". In birds the right 

 fourth arch is transformed into the permanent aorta ; and in ex- 

 amining the development of the aortic arch of the chick, it must 

 be borne in mind that it is on the opposite side to that it occupies in 

 man. 



On the right side the anterior root of the fourth arch, and the 

 part of the aortic trunk leading to it, persists as the innominate 

 artery, the fourth arch being represented by the right subclavian 

 artery. 



The part of the primitive aortic trunk joining the fourth and 

 third right anterior roots becomes the common carotid artery of 

 the same side, whilst arising from this is the internal carotid, 

 which, taking the position of the third arch, passes to the poste- 

 rior roots, and occupies the trunk of the primitive aorta from the 

 third to the first arches. 



The external carotid, arising from the common carotid at the 

 third anterior root, occupies the position of the vessel joining this 

 root to those of the second and first arch. 



On the left side the common carotid and its branches are de- 

 veloped similarly to those on the right, the only difference being 

 that the common carotid arises from the aorta and not from the 

 innominate. 



The iliac arteries are developed from the hypogastric. At 

 first they appear as branches, but with the growth of the limbs 

 they become so much larger that after birth they appear to be 

 the main branches from the point of division of the aorta, the 

 hypogastric arteries now being merely small branches of the iliac 

 vessels. 



With the development of the organs and limbs, vessels in con- 

 nection with those above described arise in the mesoblast. It is, 



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