80 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



ment (the pharynx and digestive tube), for they are only about 4^ body segments 

 apart when the arch reaches its adult position. The movement of the fourth arch 

 and later its derivative territory in the definitive aortic arch is very rapid after 

 the arches are freed from the pouches and vagus nerve, though somewhat slower 

 than the descent of the cranial end of the aorta at this time. Between the develop- 

 mental stages represented usually by embryos 14 and 18 mm. long it has moved 

 the length of about 2 vertebrse, at the rate of about one-fourth of a vertebra a day. 



The heart bears a changing relation to the sac and arches. In the first-arch 

 stage the axis of the arterial end of the heart is dorsoventral as it reaches the base 

 of the arches. Some of the truncus arteriosus is seen to approach the arches from 

 their cranial side (plate 1, figs. 30 to 32). This condition persists until near the 

 end of the branchial period, accompanied by great fluctuations of the proximal end 

 of the truncus to right or left in different individuals, as also noted in the chicks 

 by Kranichfeld (1914). In the succeeding period the trunk is reversed so as to 

 approach the arch from the caudal side (plate 3, fig. 37). At the time the change 

 takes place the division of these into aortic and pulmonary parts has permitted 

 wide separation of the lower ends of the fourth and sixth arches (plate 2, fig. 36). 

 The reason for this condition is not clear, but it is possible that as the long axis 

 of the heart swings past the perpendicular the heart may crowd the pulmonary 

 and aortic trunks, thus pushing them apart. 



The ultimate reason for the movement of the aorta and arches, judged from 

 the standpoint of individual development, is the same as already given for the 

 retreat of the cranial portion of the aorta; in each instance it is the unequal growth 

 of different organs or regions. The cranial expansion of the forward portion of the 

 central nervous system and skeleton surpasses that of the aorta, the heart, and 

 ascending part of the arch, together with certain structures lying caudal to them. 



In following the breaking down and movements of the parts of the arch system 

 various arterial changes have been described that were apparently in part due to a 

 longitudinal pull produced by the descent of the heart and the aorta. It will be 

 well to summarize these in order to better evaluate the influence of this factor on 

 arterial growth. 



The subclavians are forced to move along the arch, and the innominate and 

 common carotids are swung around into a more longitudinal position. As their 

 points of origin recede, the latter elongate rapidly, as do also the proximal portion 

 of the aortic arch and probably the corresponding portion of the early post-branchial 

 pulmonary channel. At an early stage in the descent, segmental arteries and 3 

 segments of the paired aortse contract, atrophy, stretch into long filaments, and 

 finally give way. 



The complications resulting from the lateral movement of the fourth and 

 sixth arches, due to their continuity with the aortic sac and the aorta, are interest- 

 ing. At the beginning of the post-branchial period the two pairs of arteries are 

 bent around the pouch complex and the vagus nerve, respective^, as though the 

 heart and aorta were pulling them caudally against these structures. Later, we 



