external carotid 



ductus caroticus 

 common carotid stem 



pulmonary artery ' 

 A 2 mm B 3 mm C 5.2 mm D 6 mm E 8.8 mm 



Figure 11-8. Five stages in tiie development of the aortic arches of man. (After Kramer, 1942) 



internal carotid 



3 



4 

 6 

 truncus 



of valves in the truncus is not apparent. From these rows of 

 valves the four ridges observed in the higher forms devel- 

 oped and made possible the subdivision of the truncus. 



THE AORTIC ARCHES 

 Mammals 



The pharynx of the mammal in the course of its develop- 

 ment produces a series of branchial pouches. These pouches 

 do not break through to the exterior, but they do resemble 

 the pouches which give rise to the gill clefts in fishes. Re- 

 lated to these pouches are a series of arteries, the aortic 

 arches. These connect the ventral aortic trunk (ventral 

 aorta) with the dorsal posteriorly conducting trunk (dorsal 

 aorta) or trunks (radices aortarum). 



In development the first arch is followed shortly by the 

 second (Figure 1 1-8). The arches lead into separate dorsal 

 radices which further posteriorly fuse and form the midline 

 dorsal aorta. The third and fourth arches now appear, 

 while the first and second arches undergo reduction. By the 

 time the sixth arch has appeared, the first and second are 

 greatly reduced. The third and fourth arches are large, the 

 fifth very small (Figure 11-9). The sixth sends a posterior 

 branch back to the lungs and connects with the dorsal aorta 

 by a ductus arteriosus (ductus Botalli). 



The first and second arches disappear, and their basal 

 stem becomes the external carotid (Figure 11-10). The 



ductus caroticus 

 porathyroid III 

 thymus 



ductus arteriosus 



thyroid 



pulmonary artery 



trachea 

 esophagus 



external carotid 

 parathyroid IV 



pulmonary artery. 



A3 



6 mm 



B 



7 mm 



Figure 1 1-9. Two stages in the development of the aortic arches of 

 man as seen from the side. (After Prentis and Arey, 1917) 



third arch becomes the base of the internal carotid. Dorsal 

 connection between the third and fourth arches (carotid 

 duct) is reduced and eventually lost. The left fourth arch, 

 the systemic, continues to develop, while that of the right 

 side disappears. The fifth arch disappears shortly after its 

 appearance if it appears at all. The ductus arteriosus of the 

 sixth arch atrophies at the time of birth, thus directing all 

 of the blood of this arch through the lung. In terms of the 

 adult, the systemic trunk extends to the left as the base of 

 the systemic arch. The right fourth arch remains as the 

 right subclavian artery. It has lost its connection with the 

 dorsal aorta. The right subclavian and the right common 

 carotid arise from the aortic stem by a common vessel, the 

 brachiocephalic (innominate) artery. The left common 

 carotid and subclavian arise separately from the aortic arch. 



Reptiles and birds 



The aortic arches of the reptile pass through the same basic 

 stages as those of the mammal. The primary difference lies 

 in the fact that both systemic arches are retained as con- 

 tinuations of separate divisions of the truncus (Figure 11-11). 

 The subclavian arteries pass to the limbs from the systemic 

 arches and the carotid duct is usually retained (Iguana but 

 not Varanus). The pulmonary arches are like those of the 

 mammal, the ductus arteriosus being closed at the time of 

 hatching with the result that all blood is directed to the 

 lung. The exception to this is Sphenodon which sometimes re- 

 tains this connection. 



In the bird, the reptile pattern of development is main- 

 tained The right fourth arch becomes the systemic arch, the 

 left disappears. The fifth arch, as in the mammal, appears 

 only briefly if at all (Figure 11-12). The carotid duct of the 

 left side loses connection with the left radix aortae and be- 

 comes the left ductus Shawi, which, along with the right 

 one formed later, sends branches to the syrinx, bronchi, 

 lungs, and esophagus. The left radix aortae develops a new, 

 more medial connection with the pulmonary arch as the 

 ductus arteriosus atrophies. Just before hatching, the right 

 ductus caroticus loses its connection with the systemic arch 

 and the ductus arteriosus of that side is lost (may form a 

 ligament). The left radix aortae now becomes ligamentous. 



It is usually presumed that the left systemic arch would 

 be associated with the pulmonary stem as it is in the croc- 



THE AORTIC ARCHES 



347 



