XXII. The development of the branchial arterial arches in birds, with special reference to the origin of the subclavians and carotids

According to the theories of Rathke, which are universally accepted at the present day, the subclavian artery is supposed to take its origin from the aortic root or fourth embryonic branchial arterial arch. In the adult bird the subclavian on each side is found springing from, the extremity of an innominate artery along with the common carotid. It is presumed that the right subclavian has been, by a shortening of the aortic arch, carried forwards until it meets and fuses with the base of the common carotid artery; and the left subclavian is regarded as representing by its basal portion the fourth left arch or left primitive aorta.