72 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



region. As was seen from the description of the 14-mm. embryo, the primitive 

 innominate must, from its general relationships, be largely an elaboration of the 

 right half of the aortic sac (plate 3, figs. 37, 39). It appears at this time as a trans- 

 verse tube. As the arch makes its rapid descent, this swings around to a direction 

 nearly parallel to the long axis of the body. Due to the rapid expansion of the 

 arch the innominate takes on the appearance of a branch. Measurements show 

 that its length remains about constant in the embryonic part of the post-branchial 

 phase, but that at its beginning there is evidently a period of elongation, since it is 

 longer than the right half of the sac. Its diameter equals that of the sac. In a 

 series from the post-branchial period it extends over a distance of about one and a 

 half vertebrae and is consequently much longer relatively than at maturity. The 

 chief precursor of the external carotid artery at the end of the branchial period is 

 found to be coming off from the third aortic arch near its origin. At the time 

 when the arterial territories derived from the third and fourth arches can be dis- 

 tinguished only by means of the vanishing remnants of the aorta? which lie between 

 their upper ends it is found to have shifted out upon the third arch. 



Kingsbury (1915a) has given a suggestive schema to show the influence of the 

 widening metapharynx by the successive "moving out" of the first aortic arch 

 upon the second, the second upon the third, and the third upon the fourth. If we 

 substitute the primitive external carotid for a persistent proximal end of the second 

 arch and recognize that the aortic sac itself elongates rather than that the third 

 arch moves out on the fourth, the schema is still useful as emphasizing the associa- 

 tion of the lateral movement of ventral parts of the arch system and the vessels 

 which succeed them with the lateral growth of the pharynx. 



The portion of the third arch territory proximal to the primitive external ca- 

 rotid on either side constitutes the primitive common carotid artery. This vessel, 

 like the innominate, elongates as the aortic arch moves away from the pharyngeal 

 region and swings into a more longitudinal position (plate 2, figs. 35, 36; plate 3). 

 As it passes upward, however, it still bends laterally and ventrally. This is a 

 result of the large size of the head at this time relative to the neck. It is not 

 possible to say with certainty whether the entire territory derived from the third 

 arch is ultimately to be found in the common carotid. As this vessel elongates, 

 it is possible that it also is pulled downward relative to the external carotid, so that 

 the early shifting of the latter vessel, which we can recognize up to the middle of 

 the third arch, may be continued the entire length of the arch, or even farther; 

 or it may be that such a degree of elongation is effected by the growth of the region 

 derived from the proximal half of the third arch that the external carotid does not 

 shift beyond the middle of the region derived from the arch. 



The late history of the right fourth aortic arch and the part of the right paired 

 aorta caudal to it is bound up in the development of the right subclavian artery. 

 The interruption of the left paired aorta cranial to the fourth arch and distal to the 

 subclavian permits a swinging around of the arch and the remaining division of the 

 aorta until they are aligned with the primitive subclavian (figs. 13 to 16). These 

 changes will be more fully explained in the history of the subclavian arteries. 



