68 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



merits. In the further examination of the growth of the aortic arch it is to be re- 

 membered that there are three regions of the arch system to consider the aortic 

 trunk, the left half of the aortic sac, and the left fourth arch together with the paired 

 aorta between it and the pulmonary arch (plate 2). These parts are to be compared, 

 respectively, with the proximal end of the arch from valves to innominate artery 

 (plate 3), the portion between the innominate and left common carotid, and the part- 

 between the left common carotid and the ductus arteriosus. The part of the left 

 aorta which enters into the formation of the arch was not especially studied. 



The distance from aortic valves to the left pulmonary arch, or, later, to the 

 ductus arteriosus, which includes nearly all of the arch, does not increase from the 

 late branchial period to the stage represented by a 24-mm. embryo with sternal bands 

 in contact and the heart and large vessels in nearly their adult thoracic position. 

 There is no reason for believing that this failure to elongate is only apparent and due 

 to a proximal movement of the aortic ductus. If such a shifting should take place, 

 it would naturally be greatest at the time of rapid descent, yet no change in the 

 distance from the valves occurred at this time. Doubtless, then, there is a true 

 standstill in longitudinal growth. 



Though the arch does not elongate, it does increase in diameter. The measure- 

 ments show that the left fourth arch and, to a less degree, the left paired aorta in- 

 crease rapidly in circumference as the aortic arch is forming. The sac region of the 

 arch alone is much larger around in the post-branchial period than is the sac in the 

 branchial period. By these enlargements an arch is developed without local inequal- 

 ities and with connection adequate to carry more than half of the entire current to 

 the dorsal aorta, which was formerly divided between six branchial aortic arches. 

 The changes in extent of the divisions of the arch will be best understood if the inter- 

 val between the innominate and left common carotid be first considered. In the 

 early post-branchial period this is somewhat greater than the length of the left half 

 of the aortic sac, to which it was equivalent at the beginning of the period. It 

 reaches a maximum at about the time of the rapid descent of the arch (16 to 17 mm. 

 embryos) and decreases rapidly while the rudiments of ribs and sternum are closing 

 in to form the superior thoracic aperture. The increase in length indicates a real 

 growth, since the circumference of this region does not decrease, and it is evident 

 that the innominate and left common carotid rather precisely mark off territory 

 derived from the earlier left half of the sac during the first part of post-branchial 

 development and are withdrawing from each other at this time because the part of 

 the arch between their points of origin is conforming to the general body-growth. 

 The later approach of the two branches in embryos of 18 to 24 mm. length must be 

 due to a different process in the wall of the arch, for the increase in the circumference 

 at this time is not nearly as great as the decrease in distance between the two ar- 

 teries. Hence we can not explain their approach on the basis of a mere reshaping of 

 the wall of the arch between them by which it gains in circumference what it loses in 

 length ; there must have been an actual decrease in the substance of the wall of that 

 part of the arch or a plastic rearrangement, allowing the vessels to approach by one 



