76 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



In the cervical region in a 12-mm. embryo the paired aortae lie in a little groove 

 on either side at the plane of contact of the condensed mesenchyme of the vertebrae 

 and the esophagus. It is apparently the expansion of these masses that has carried 

 them apart. In the anterior pharyngeal region the great widening at the level of 

 the first and second pouches has carried the aorta with it just as it has carried apart 

 the first and second aortic arches on the ventral side of the pharynx. In the early 

 part of the branchial period the first and second arches probably aid in the separa- 

 tion by holding the aortae close to the lateral borders of the pharynx as it widens. 

 The most cranial divergence of the paired vessels is in the territory of the longitudi- 

 nal neurals and is the result of the growth of the forebrain, upon which they lie. 

 On plate 2 (figs. 33 and 35) are shown the caudal and intermediate curves. 



The regions of approximation of the aortae are interesting, since they must cor- 

 respond to territories of sluggish lateral growth in the environment. The more 

 caudal of these is at the esophageal end of the pharynx, and therefore includes the 

 attachment of the fourth and pulmonary arteries. It is not surprising that growth 

 should be slight here, since this division of the pharynx, as is well known, shows 

 many regressive features. It is of interest that it has not only affected the course of 

 the aortae because of this characteristic but, as previously seen, has prevented any 

 considerable growth in length on the part of the more caudal aortic arches. The 

 point of greatest approximation is just caudal to the pharynx, and it is exactly here 

 that in the beginning of the post-branchial period the vagus nerve often leaves an 

 impression on the aortae as it curves around their outer surface in its caudal and 

 ventral course to lungs and digestive tract. It may be that the nerve exerts a minor 

 influence in maintaining a close approximation of the two vessels. 



The proximity of the arteries just in front of the pharynx indicates that the 

 mesenchyme here has not expanded laterally as fast as the pharynx behind and the 

 forebrain in front. There has been some separation of the vessels such as one would 

 expect as an expression of the tendency of any growing vessel to straighten its 

 tortuosities through the action of hydrodynamic factors. It may be that failure of 

 the artery to grow as fast as the nerve-tube and pharynx may have assisted in 

 decreasing the curvature engendered by the longitudinal tension. 



The last important lateral displacement of the aorta is the movement of the 

 entire thoracic aorta from the mid-line to a position more or less completely over to 

 the left surface of the vertebral body. A lateral shifting at the bifurcation begins to 

 show itself as soon as the right paired aorta has begun to decrease in volume relative 

 to the opposite vessel. In a 50-mm. fetus we find the aorta and the esophagus both 

 in contact with the vertebra in the thoracic region and lying to either side of the 

 mid-sagittal plane. While the lateral movement at its beginning is, to a certain 

 degree, a mere straightening of the angle between the left paired and the unpaired 

 aortae, due to hydrodynamic forces or longitudinal tension resulting from inequality 

 in growth between the aorta and its surroundings, most of the displacement is 

 doubtless the result of pressure from the vertebral column above and esophagus 

 below. It is the same process which already has been found to cause the separation 

 of the paired aortae in the region just cranial and is doubtless due to the same causes. 



