78 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



the involution of the right pulmonary arch, in which the bifurcation is at rest. 

 As has been said, it lies at this time opposite the seventh body segment. In the 

 early branchial period, as the arches are freed for further backward progress by 

 the caudal shifting of the derivatives of the pharyngeal pouches and the successive 

 interruption of various parts of the arch system, the cranial portion of the unpaired 

 aorta itself moves caudally relative to the adjacent digestive and respiratory 

 organs and the more distant organs as well. This is best shown by the movement 

 of the aortic bifurcation. 



The paired aorta is followed in its descent by the left unpaired aorta only. 

 The right, fixed by its subclavian branch, gives way in a short terminal segment 

 between subclavian and bifurcation in a manner previously described. The 

 process is just beginning in one 16-mm. embryo of our series, while in another of 

 the same length the segment has stretched to a thread whose caudal termination 

 shows the point of bifurcation to have descended from a region opposite the sixth 

 cervical to the second thoracic vertebra (fig. 14). Thyng (1914) also finds it 

 here in a 17-mm. embryo. It is probable that a rather common type of anomalous 

 subclavian described in the adult indicates roughly by its origin the ultimate 

 position of the region corresponding to the former bifurcation. It is characteristic 

 of these anomalous vessels that they pass between vertebral column and esophagus 

 and come off as the most distal branch of the arch, if, indeed, they do not arise 

 from the descending aorta itself. Their existence is probably due to the fact that 

 in their development they tap the main stream through the caudal end of the left 

 paired aorta instead of making use of the right aorta and the fourth arch. Sub- 

 clavian of this kind are found in the adult arising from the termination of the 

 arch or the aorta as far caudal as the fifth thoracic vertebra. Since the subclavian 

 and other branches of the arch shift cranially upon it, there is a possibility that the 

 aortic wall derived from the earlier region of bifurcation lies still lower. Granted 

 that the region of bifurcation in the adult lies at the sixth thoracic, the distance 

 at this time between it and the ligamentum arteriosum, which succeeds the arterial 

 duct, can not be more than the length of 3 body segments. In the branchial period 

 the bifurcation lies about 5 segments behind the pulmonary arch, as determined 

 on models of 10 embryos. There is, then, during development, a relative shortening 

 of the part of the definitive aorta derived from the left paired aorta. Since it has 

 been found that the distal part of the aortic arch, and probably also the distal part 

 of the main pulmonary channel, lags in growth behind the proximal part during 

 the early post-branchial period, it can now be said that the proximal part is in 

 contrast to the aorta as well as to the distal part. This contrast in growth in the 

 different parts of the chief arterial trunks leading from the heart is an interesting 

 condition. Perhaps it should be regarded as illustrating an accelerating effect 

 of increased longitudinal tension upon the growth of the arteries due to the descent 

 of the heart. 



It is well established that the caudal end of the aorta withdraws cranially. 

 Since the two ends approach each other, there must be a region not far from the 

 thoracico-abdominal boundary where there is little shifting in either direction. 



