70 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



In its change of position the left subclavian involves both the arch and the 

 aorta and helps one to understand the manner of their growth. The interval 

 between the left subclavian and left common carotid, as also its approximate equiv- 

 alent in the branchial period, shows a marked decrease not only relatively to body 

 length but absolutely. In fact, it is only one-fifth as long in a 24-mm. embryo as in 

 the late branchial phase. If we subtract from it the length of its proximal part as 

 far as the ductus arteriosus, it decreases to zero, since the subclavian shifts upon the 

 aorta and arch upward past the ductus. 



At its first appearance the subclavian arises from the unpaired aorta. It passes 

 the bifurcation of the aorta early in its development and on to the left paired aorta. 

 Its movement past the fusion point of the aortae and the ductus arteriosus can only 

 be explained by a considerable shifting of the material of the wall of aorta and arch, 

 and in this respect it resembles the changes in position of the innominate (figs. 18, 19, 

 22, 23). A similar condition has been found in the large abdominal arteries. Evans 

 (1912) suggests that their movement along the dorsal aorta may be due to an-unequal 

 growth of the dorsal and ventral walls. The exact nature of the translocation of 

 material which permits such shifting, however, seems to be at present very uncertain. 



To summarize the observations on the growth of the definitive aortic arch during 

 the period of rapid descent of heart and arteries and the coming together of the 

 sternal bands, before the rapid descent the proximal part of the arch extending up to 

 the origin of the left common carotid elongates rapidly and increases moderately in 

 diameter. The more distal region, as far as the ductus arteriosus, decreases in 

 length. It increases rapidly in diameter, however, to compensate for its originally 

 small cross-section as compared with the more proximal parts. Increase in length 

 or diameter, if any, during the rapid descent, is too slight to be distinguished. The 

 chief changes are in the movement of the innominate and the subclavian along the 

 wall of the arch. The innominate moves up to the left common carotid, and the 

 subclavian approaches it from the other side. The subclavian passes the ductus 

 arteriosus but does not approach very close to the carotid at this time. The large 

 part of the arch extending down to the ductus arteriosus does not increase in length 

 during the considerable developmental interval included in this study, though its 

 diameter enlarges. 



The history of the main post-branchial pulmonary channel illustrates the same 

 growth processes observed in the development of the arch. The first step is the 

 separation of the pulmonary trunk and its pair of arches from the aortic trunk 

 and sac (fig. 17a). Because the pulmonary arches arise from the sac close to the 

 mid-sagittal plane, little of the sac is removed when they separate off, and no 

 attempt will be made to trace the small zone derived from it in the later develop- 

 ment. The proximal part of the right pulmonary arch remains as the origin of 

 the right primitive pulmonary artery after its distal portion degenerates. Relieved 

 of the longitudinal tension exerted by the complete arch, the angle between the 

 remaining part of the arch and the primitive pulmonary artery tends to straighten 

 out, aided, no doubt, by a formative action of the current not associated with 

 longitudinal tension, so that the boundary between the two can no longer be 



