DEVELOPMENT OF THE BLOOD- VASCULAR SYSTEM 



765 



the sixth left arch gives off the pulmonary arteries and forms the ductus arteriosus; this duct 

 remains pervious throughout fetal life, but becomes impervious a few days after birth. 1 



The Dorsal Aortae. Cephalad of the third aortic arches the dorsal aortae persist and form 

 the headward continuation of the internal carotid arteries. Caudad of 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 which intervenes between the third and fourth arches 

 disappears, while the remainder persists to form the descending part of the arch of the aorta. 

 A constriction, the aortic isthmus, is sometimes seen in the aorta between the origin of the left 

 subclavian and the attachment of the ductus arteriosus. Below this isthmus the aorta expands 

 slightly to form the aortic spindle. 



Sometimes the right subclavian artery arises from the aortic arch beyond the origin of the left 

 subclavian and passes upward and to the right behind the trachea and oesophagus. This con- 

 dition 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 stoma- 

 todeum. With the elongation of the neck and development of the lungs it recedes within the 

 thorax, and, as a consequence, the anterior ventral aortse 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 



Right subclavian 

 artery 



Right pulmonary 

 artery 



Trunk of pulmonary 

 artery 



-External carotid 

 Ventral aorta 



Internal carotid 



Common carotid 

 Aortic arch 



Ductus arteriosus 

 Vertebral artery 



Subclavian artery 

 Left pulmonary artery 



FIG. 544. Scheme of the aortic arches and their derivatives. (Modified from Kollmann.) 



the root of the neck, while on the left side it is withdrawn within the thorax.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



laryngeal nerves originally pass to their distribution under the sixth pair of arches, and are 



I therefore pulled backward with the descent of these structures, so that in the adult the left hooks 



j around the ductus arteriosus; owing to the disappearance of the fifth and the sixth right arches 



i the right nerve hooks around that immediately above them, i. e., the commencement of the sub- 



! clavian artery. A series of segmental arteries arises from the primitive dorsal aortse, those in 



! the neck alternating with the cervical segments of the vertebral column. The seventh segmental 



artery, which lies between the sixth and seventh cervical segments, is of special interest, since it 



: forms the lower part of the vertebral artery and, when the forelimb bud appears, sends a branch 



' to it (i. e., the subclavian artery); the upper part of the vertebral artery is formed by an inter- 



segmental anastomosis between the higher segmental arteries. From the seventh segmental 



arteries the entire left subclavian and the greater part of the right subclavian are formed. 



The subclavian artery is prolonged into the limb under the names of the axillary and brachial 

 arteries, and these together constitute the arterial stem for the upper arm. The direct con- 

 tinuation of this stem into the forearm forms the anterior interosseous artery; while the radial 

 anil ulnar vessels, which ultimately exceed this artery in size, are in reality lateral branches of the 

 i main stem. 



The formation of the primary caudal branches has already been referred to (p. 758), and the 

 fusion of the dorsal aortae to form the greater part of the systemic aorta has been pointed out 



1 His found that in the young embryo the right and left sixth arches each give off a branch to the lungs, but 

 that later both pulmonary arteries take origin from the left arch. 



