82 



AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



a vertebra length more caudally. The larger and more rapidly moving arch has 

 apparently aided in the movement of the pharyngeal tissue which lies in its path. 

 Kingsbury has pointed out that the shifting of even the pharyngeal derivatives 

 is in part due to their own outgrowth. 



In a succeeding discussion, based on models prepared to show the rudimentary 

 ribs and sternal bands as well as arteries, it will be seen that the changes in the 

 ribs are also somewhat suggestive of material in the growth eddy. 



RELATION OF MIGRATING ARCH AND ITS BRANCHES TO SUPERIOR APERTURE 



OF THORAX. 



The approach and entrance of the heart and its arterial vessels into the 

 thorax is characterized bj r a nice coordination between the time of arrival of the 



Vertebral art. 



Vertebral art 



Common carotid art. 



Innominate art. 

 Subclavian art.. 

 Da 



Figs. IS to 2.5. The descent of the fourth aortic arch and the definitive aortic arch into the thorax, shown in relation 

 to the cervical vertebra? and ribs. Asterisk, so-called fifth aortic arch; 4, fourth aortic arch; d. a., 

 definitive aortic arch; R. 1, first rib; in. art., innominate artery; c. r., cervical rib; c. c, common 

 carotid; v. art., vertebral artery; a. r., remnant of segment of dorsal aorta, interrupted between third 

 and fourth aortic arches; sub. art., subclavian artery. In figures 20 and 24 the sternal bands are not yet 

 in contact above and the definitive aortic arch has a large radius of curvature. In figures 21 and 25 

 the bands have met and the arch has become sharply bent by the swinging dorsally of the heart. 



heart and aortic arch at the thorax and the coming together of the ribs and sternum 

 in front. Within an interval of 10 days the upper ribs on each side, capped by their 

 sternal bands, have completed the thoracic arch (figs. 18 to 25). 



In the 14-mm. embryo the ribs are slightly concave cranially and nearly straight 

 in the transverse plane. By the time the embryo is 24 mm. long they have grown 

 forward and around so that the sternal bands capping their tips are fusing in the 

 mid-line. Three models between the earliest and latest stages of this series 

 show the rib as a whole sloping cranio-ventrally, but at 24 mm. they are once more 

 horizontal. Between the earliest and latest stages the rib elongates about three- 

 fold. It grows forward and medially, reshapes itself, and expands. It gives the 



