AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 103 



at the summit of the forming definitive arch, sinks to the descending limb, and the 

 region of the left common carotid, now shared by the innominate, comes to con- 

 stitute the entire summit. The region of the arch derived from the fourth arch 

 increases rapidly in diameter to reach in cross-section an approximate equality 

 with the more proximal and distal parts derived from vessels which are already 

 capacious at the beginning of the branchial period. The arch as a whole increases 

 little in length, but considerably in circumference, up to the 24-mm. stage. 



The definitive arch in its early history lies in almost the mid-sagittal plane, 

 because at its distal end the aorta has not taken a paravertebral position and at its 

 proximal end the heart has not yet come to he obliquely in the thorax. The arch 

 has a large radius before the closing of the superior thoracic aperture by the meeting 

 in the front of sternal bands and ribs. Then, due to the swinging caudally of the 

 apex of the heart to accommodate itself to the decreased space of the thoracic 

 cavity, it is bent rather sharply at its summit. 



The innominate and common carotids swing into a nearly longitudinal position 

 during the rapid descent. They still slope somewhat ventrolaterally in the 24-mm. 

 embryo as they pass upward, because of the large size of the head relative to the 

 body. The innominate lengthens to about the same degree as it decreases in cir- 

 cumference ; relative to the increasing body-length it is much longer proportionally 

 in the 24-mm. embryo than it is in the adult. The common carotid arteries extend 

 rapidly coincident with the rapid descent of the heart. It is not clear how much 

 of this is due to the elongation of the region from the proximal half of the third arch 

 and how much to the arch being pulled caudal ward, thus forcing the external 

 carotid to shift cranially along its wall and the wall of the dorsal aorta cranial to it. 



Changes in Topography of Aortic-Arch System. 



The displacements of the parts of the arch system and of the aorta, due to the 

 unequal growth of different organs, are chiefly longitudinal and transverse. The 

 paired primitive aortse grow toward each other in a part of their course and are 

 carried apart in other regions. The approach is in the thoracic region and is not 

 a movement of the vessels as a whole toward each other, but merely an approxima- 

 tion of then contiguous walls due to the increase in diameter of the vessels. It is 

 permitted by the withdrawal of the nerve-tube and notochord from the digestive 

 tract. 



The fusion of the aorta? takes place by the enlargement of capillaries lying 

 between the vessels to form transverse anastomoses. These then fuse so that a 

 unit vessel results with a cross-section like the figure 8. This in turn is remolded to 

 the ordinary arterial form. The fusion begins somewhat back of the cervical region 

 and progresses both cranially and caudally. It comes to a stop about 4J^ body 

 segments caudal to the pharyngeal territory, where the pulmonary arch is forming. 

 Due to the growth displacement of the cranial end of the nerve-tube relative to the 

 pharynx, the most cranial point of fusion soon after the process has begun is opposite 

 the second cervical ganglion; while later, when fusion is complete, though it has 

 moved forward relative to the pharynx, it is opposite the seventh ganglion. 



