AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 77 



It is probable that the initial deflection of its upper end, due to the retention of 

 the left instead of the right paired aorta, is the cause of its slipping to the left rather 

 than the right. It is of interest in this connection that Krause (1868) , in his discussion 

 of arterial anomalies, states that the retention of the right paired aorta and right 

 arch is frequently accompanied by a dextral position of the thoracic aorta. 



The changes in position of the aortse along the transverse axis may be classified, 

 therefore, as of three kinds: (1) a further separation in the middle cervical and 

 anterior pharyngeal regions, with which may be grouped a separation of the paired 

 longitudinal neural arteries under the forebrain; (2) an approximation of contiguous 

 surfaces due to growth of the vessels in caliber, chiefly in the thoracic region, which 

 results in their fusion by means of anastomoses ; (3) a translocation of the thoracic 

 and abdominal aortse toward the left side of the vertebral column, due to the 

 pressure of structures lying dorsal and ventral to it. 



MIGRATION OF AORTIC-ARCH SYSTEM. 



LONGITUDINAL SHIFTING OF AORTA. 



It would be easy to interpret the cranial elongation of the region of fusion of 

 the aortse as a cranial shifting of the unpaired vessels did not the presence of trans- 

 verse communications and peculiarities in the form of the cranial end of the fused 

 region point to its true nature. The true caudal shifting of the aortse begins before 

 fusion is complete; yet there is no reason for confusion of the two processes, since 

 it is only the cranial end of the paired vessels that is at this time involved. 



The moving of the aorta relative to its surroundings is progressive, beginning 

 in the region of the first aortic arch, perhaps even farther forward, and gradually 

 extending to more cranial parts of the vessel. There can be no doubt that it is due 

 to a slowing down of the longitudinal growth relative to the pharynx and digestive 

 tube, and this must first take place only at the cranial end, later manifesting 

 itself in regions progressively more caudal. 



The first indication of the caudal movement is the shifting of the third aortic 

 arch from a position at the middle of its visceral arch to its most caudal border and 

 the bending backward of its upper end before entering the aorta (plate 2, fig. 34). 

 These changes are in turn followed by the other arches, until, in the late post- 

 branchial period (plate 2, fig. 36), even the pulmonary arch, as we have seen, 

 bends markedly backward at its upper end before entering the aorta. At this 

 time, also, the shifting can be seen, by the sharp caudal bend of the proximal end 

 of the more cranial cervical segmentals, to have proceeded beyond the pharynx 

 (fig. 27) . The more moderate cranial slope of the distal part of each of these arteries 

 is due to another cause, namely, the shifting of the nerve-tube relative to the 

 digestive tract, which forces an oblique direction not only on the part of these 

 vessels but also on other structures of the body segments lying between them. 



The aortic region involved in the shifting does not extend ^to the bifurcation 

 until the end of the branchial period. There is therefore a considerable interval 

 of time, beginning with the completion of the pulmonary arches and extending to 



