AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 79 



No connected account of the shifting of the dorsal aorta is to be found in the 

 literature. Goette (1875) has noted in the toad the retreat of the bifurcation 

 point of the aorta, and Hochstetter (1890) finds it to be a regular occurrence in 

 amniote development. Both observers fall into the error of regarding it as the 

 result of splitting. Hochstetter further states that it results in the lengthening 

 of the aortic roots, which term he applies to the paired aortse caudal to the pul- 

 monary arch. In man, at least, as has just been seen, their change in length is 

 of the opposite kind. 



SHIFTING OF ARCHES AND THEIR VENTRAL CONNECTIONS. 



The movement of the dorsal aortse is but part of the general descent of the 

 cervical viscera into or towards the thorax. The heart and aortic arches not only 

 share in the movement but are not exceeded by any other structure in the distance 

 covered. The shifting of the aortic sac in the branchial period is slow and cor- 

 responds in amount to the aortic displacement at this time. 



The sac moves backward along the floor of the pharynx, keeping pace with the 

 appearance of new caudal aortic arches and the disappearance of the more cranial 

 ones. Kingsbury shows that the apparent distance it has moved is enhanced 

 by the active forward growth of the anterior pharyngeal region. It is not clear 

 whether the movement of the sac at this time is a translocation of the entire struc- 

 ture relative to the pharynx or a mere growth backward of its caudal portion by 

 the development of successive ba} r s which take part in the formation of the arches 

 as they appear one after another. The constant position of the trunk relative 

 to the sac speaks for the former view. In the post-branchial period there can be 

 no doubt of a translocation of the sac. The extent of this journey may be learned 

 from the succeeding account of the migration of the fourth arch, since the two 

 move approximately the same distance. 



The movement of the more caudal aortic arches through their visceral arches 

 has already been referred to. At the end of the branchial phase of development 

 the arches present at that time are hooked around the structures which are appar- 

 ently preventing their caudal progress. The pharyngeal pouches are in the way of 

 the two more cranial arches, while it is the vagus and recurrent nerves which seem 

 to bar the way of the pulmonary arches. After their development from the 

 pouches the pharyngeal derivatives lose their connection with the pharynx, thus 

 removing the obstacle to the migration of the third and fourth arches. The right 

 pulmonary arch, being under less favorable conditions of current-flow than its 

 mate, undergoes degeneration. The left persists and apparently forces the recur- 

 rent nerve of its side to elongate in order to give way to its advance. 



Sufficient for an illustration of the shifting of the arches is the left fourth arch, 

 which gives rise to a zone of the definitive arch. At its first appearance it is below 

 the first occipital segment, while in the adult the definitive arch overlaps the second 

 and third vertebra?, and the zone of the fourth is caudal to the summit of the arch. 

 The fourth arch and its derivative tissue therefore shift the length of 13 body 

 segments (figs. 22 to 25), but not nearly so far in relation to the immediate environ- 



