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DEVELOPMENT OF THE VASCULAR SYSTEM 517 



and the termination of the ductus arteriosus. The fifth arch disappears on both 

 sides. The sixth right arch disappears; the sixth left arch gives off the pulmonary 

 arteries and forms the ductus arteriosus; this duct remains pervious during the 

 whole of fetal life, but is obliterated a few days after birth. His showed that in the 

 early embryo the right and left arches each gives a branch to the lungs, but that 

 later both pulmonary arteries take origin from the left arch. 



The Dorsal Aortse.- In front of the third aortic arches the dorsal aortse persist 

 and form the continuations of the internal carotid arteries; these arteries pass to the 

 brain and each divides into an anterior and a posterior branch, the former giving 

 off the ophthalmic and the anterior and middle cerebral arteries, while the latter 

 turns back and joins the cerebral part of the vertebral artery. Behind the third 

 arch the right dorsal aorta disappears as far as the point where the two dorsal 

 aortse fuse to form the descending aorta. The part of the left dorsal aorta between 



e third and fourth arches disappears, while the remainder persists to form 



e descending part of the arch of the aorta. A constriction, the aortic isthmus, is 

 ometimes seen in the aorta between the origin of the left subclavian and the 

 attachment of the ductus arteriosus. 



Sometimes the right subclavian artery arises from the aortic arch distal to the 

 origin of the left subclavian and passes upward and to the right behind the trachea 

 and esophagus. This condition may be explained by the persistence- of the right 

 dorsal aorta and the obliteration of the fourth right arch. 



In birds the fourth right arch forms the arch of the aorta; in reptiles the fourth 

 arch on both sides persists and gives rise to the double aortic arch in these animals. 



The heart originally lies on the ventral aspect of the pharynx, immediately 

 behind the stomodeum. With the elongation of the neck and the development 

 of the lungs it recedes within the thorax, and, as a consequence, the anterior 

 ventral aortas are drawn out and the original position of the fourth and fifth arches 

 is greatly modified. Thus, on the right side the fourth recedes to the root of the 

 neck, while on the left side it is withdrawn within the thorax. The recurrent 

 nerves originally pass to the larynx under the sixth pair of arches, and are there- 

 fore pulled backward with the descent of these structures, so that in the adult the 

 left nerve hooks around the ligamentum arteriosum; owing to the disappearance of 

 the fifth and the sixth right arches the right nerve hooks around that immediately 

 above them, i. e., the commencement of the subclavian artery. Segmental arteries 

 arise from the primitive dorsal aortas and course between successive segments. 

 The seventh segmental artery is of special interest, since it forms the lower end of 



e vertebral artery and, when the forelimb bud appears, sends a branch to it 

 (the subclavian artery). From the seventh segmental arteries the entire left 

 subclavian and the greater part of the right subclavian are formed. The second 

 pair of segmental arteries accompany the hypoglossal nerves to the brain and are 

 named the hypoglossal arteries. Each sends forward a branch which forms the 

 cerebral part of the vertebral artery and anastomoses with the posterior branch 

 of the internal carotid. The two vertebrals unite on the ventral surface of the 

 hind-brain to form the basilar artery. Later the hypoglossal artery atrophies 

 and the vertebral is connected with the first segmental artery. The cervical part 

 of the vertebral is developed from a longitudinal anastomosis between the first 

 seven segmental arteries, so that the seventh of these ultimately becomes the source 

 of the artery. As a result of the growth of the upper limb the subclavian artery 

 increases greatly in size and the vertebral then appears to spring from it. 



Recent observations show that several segmental arteries contribute branches to 

 the upper limb-bud and form in it a free capillary anastomosis. Of these branches, 

 only one, viz., that derived from the seventh segmental artery, persists to form 

 the subclavian artery. The subclavian artery is prolonged into the limb under 

 the names of the axillary and brachial arteries, and these together constitute the 





