AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 71 



identified. The loss of this tension at the junction of the two pulmonary arches, 

 taken similarly with the action of the increasing current, permits the pulmonary 

 trunk and the left pulmonary arch to align (fig. 17, a to d). The resulting straight 

 vessel is the main pulmonary channel and carries the blood from the right ventricle 

 to the aortic arch until its distal end, the ductus arteriosus, becomes closed soon 

 after birth. The proximal segment of the right arch, now part of the right pul- 

 monary artery, is still present to mark more or less definitely the zone corresponding 

 to the earlier point of origin of the pulmonary arches. An idea as to how long 

 the vessel will serve this purpose may be obtained from the changes in dimensions 

 of the divisions of the pulmonary channel which it subtends. 



There are three territories of the arch system to trace into the later pulmonary 

 vessels: the pulmonary trunk from the valves to the origin of the pulmonary arches, 

 the proximal part of the left arch up to the origin of the left primitive pulmonary 

 artery, and the distal part of the arch from the artery to its upper end (plate 2). 

 They are to be compared, respectively, with the later distance from the pulmonary 

 to the origin of the right pulmonary artery, the interval between the origins of the 

 two pulmonary arteries, and the length of the ductus arteriosus (plate 3). 



The segment from valves to right pulmonary artery elongates during the 

 transition from branchial to post-branchial phase. It increases as rapidly as the 

 body length during the earlier part of the post-branchial period. The interval 

 between the two primitive pulmonary arteries remains for a time about equal to 

 the earlier segment of the left pulmonary arch up to the origin of the left pulmonary. 

 During the rapid descent, however, the two vessels approach, and before a length 

 of 40 mm. is attained they come off side by side. There is also no increase in the 

 length of the ductus arteriosus over the part of the left arch distal to the origin of its 

 pulmonary artery. From the late branchial period to the end of the period under 

 consideration the ductus decreases to one-fifth of its former extent relative to body 

 length. 



The fact that there is an increase in length in the region of the main pulmonary 

 channel proximal to the two pulmonary arteries and a decrease in the portion distal 

 to them suggests the possibility that the points of origin of the two vessels shift 

 distally. At least while they are approaching each other, one or both of them must 

 move through the wall. However, a large part of the increase in the length of the 

 proximal division of the channel and the decrease of the ductus arteriosus occurs 

 before the distance between the two pulmonary arteries begins actually to decrease. 

 It is probable that at this time inequalities in longitudinal growth between these 

 two terminal segments are the chief if not the sole cause of the shifting of the 

 arteries. If this be true, in spite of the great decrease in length of the ductus 

 arteriosus relative to body length in the late branchial and the early post-branchial 

 periods, increase of its wall substance must still have been taking place, because in 

 this period its circumference is greatly augmented, ffy the rapid decrease in 

 relative length the ductus is approaching the small size, relative to adjacent parts, 

 which it maintains throughout its later existence. 



The innominate and common carotid arteries change rapidly into long trunks 

 as the aortic arch shifts caudally from the branches of the carotids in the head 



