106 AORTIC- ARCH SYSTEM IN THE HUMAN EMBRYO. 



however, probably have a more active role, and they themselves, wliile pulled by 

 the heart, actually aid the caudal movement of some of the other structures by 

 pressure against them. The moving in of structures to take the place of the heart 

 has been aptly termed by Kingsbury a "growth eddy." The body-wall takes an 

 active part in the eddy, as indicated by the change in position of the rib rudiments 

 and sternal bands. The ribs, before the rapid descent, point upward and outward 

 at an angle of 90 with the sagittal plane. They sink caudally at their distal ends, 

 and by the development of a curve and an increase in length they come to form, 

 with the aid of the sternal bands, an arch which completes the thoracic inclosure 

 on its ventral side. This process follows quickly upon the descent of the heart 

 into the thorax. 



The heart and large vessels change their position just in time to accommodate 

 themselves to the restricted quarters resulting from the closure of the superior 

 thoracic aperture. The kinking of the aortic arch, which occurs at this time, results 

 in a dorsoventral diameter commensurate with the size of the aperture. The left 

 subclavian and the innominate are now near the summit of the arch, so that the 

 arches of the branch are well placed to find exit from the thorax. 



The arterial changes that have been recounted include many illustrations of 

 apparent effects of the longitudinal pull of the heart and dorsal aorta on the arteries 

 with which they are connected, usually acting alone but sometimes associated with 

 other causes. Among these may be mentioned the involution, the stretching into 

 threads and the breaking of segments of the arch system, the caudal movement of 

 vessels, swinging of vessels into an approximately longitudinal direction, especially 

 rapid growth of arteries during the descent of the definitive arch, and the movement 

 of the subclavian and innominate along the vessels of origin. Experimental evi- 

 dence will be necessary to establish definitely the action of longitudinal tension in 

 most of these cases, but even in the absence of light from this quarter the develop- 

 mental picture offers strong indications in its favor. 



Pulmonary Artery. 



The pulmonary artery takes origin when the sprout from the dorsal aorta 

 caudal to the caudal pharyngeal complex establishes a connection with the caudally 

 extending sprout from the aortic sac, thus dividing the latter into two parts a 

 proximal portion, which becomes part of the pulmonary arch, and a distal portion, 

 the primitive pulmonary artery. The sprout from the sac is preceded by a well- 

 developed plexus, which itself has sprung from the sac and seems more to be the 

 result of the elaboration of the endothelium of the plexus than were the other 

 aortic-arch sprouts. 



When the right pulmonary arch becomes interrupted dorsal to the origin of 

 the right pulmonary artery, the angle between the artery and the proximal segment 

 of the arch straightens out, so that the arch remnants become a part of the artery. 

 Similarly, the angle between the left arch and pulmonary trunk becomes rectilinear, 

 so that these two elements form a large trunk, slightly curved^which extends from 

 the pulmonary side of the heart to the distal end of the aortic arch. In the straighten- 



